$\displaystyle P=2l+ 2w$

We have the perimeter and the width, so we can plug those values into our equation and solve for our unknown. 

$\displaystyle 72=2l+2(15)$

$\displaystyle 72=2l+30$

Subtract $\displaystyle 30$ from both sides

$\displaystyle 72-30=2l+30-30$

$\displaystyle 42=2l$

Divide $\displaystyle 2$ by both sides

$\displaystyle \frac{42}{2}=\frac{2l}{2}$

$\displaystyle 21=l$

$\displaystyle P=2l+ 2w$

We have the perimeter and the width, so we can plug those values into our equation and solve for our unknown. 

$\displaystyle 12=2l+2(3)$

$\displaystyle 12=2l+6$

Subtract $\displaystyle 6$ from both sides

$\displaystyle 12-6=2l+6-6$

$\displaystyle 6=2l$

Divide $\displaystyle 2$ by both sides

$\displaystyle \frac{6}{2}=\frac{2l}{2}$

$\displaystyle 3=l$

$\displaystyle P=2l+ 2w$

We have the perimeter and the width, so we can plug those values into our equation and solve for our unknown. 

$\displaystyle 24=2l+2(3)$

$\displaystyle 24=2l+6$

Subtract $\displaystyle 6$ from both sides

$\displaystyle 24-6=2l+6-6$

$\displaystyle 18=2l$

Divide $\displaystyle 2$ by both sides

$\displaystyle \frac{18}{2}=\frac{2l}{2}$

$\displaystyle 9=l$

$\displaystyle P=2l+ 2w$

We have the perimeter and the width, so we can plug those values into our equation and solve for our unknown. 

$\displaystyle 26=2l+2(5)$

$\displaystyle 26=2l+10$

Subtract $\displaystyle 10$ from both sides

$\displaystyle 26-10=2l+10-10$

$\displaystyle 16=2l$

Divide $\displaystyle 2$ by both sides

$\displaystyle \frac{16}{2}=\frac{2l}{2}$

$\displaystyle 8=l$

$\displaystyle P=2l+ 2w$

We have the perimeter and the width, so we can plug those values into our equation and solve for our unknown. 

$\displaystyle 18=2l+2(3)$

$\displaystyle 18=2l+6$

Subtract $\displaystyle 6$ from both sides

$\displaystyle 18-6=2l+6-6$

$\displaystyle 12=2l$

Divide $\displaystyle 2$ by both sides

$\displaystyle \frac{12}{2}=\frac{2l}{2}$

$\displaystyle 6=l$

$\displaystyle P=2l+ 2w$

We have the perimeter and the width, so we can plug those values into our equation and solve for our unknown. 

$\displaystyle 30=2l+2(8)$

$\displaystyle 30=2l+16$

Subtract $\displaystyle 16$ from both sides

$\displaystyle 30-16=2l+16-16$

$\displaystyle 14=2l$

Divide $\displaystyle 2$ by both sides

$\displaystyle \frac{14}{2}=\frac{2l}{2}$

$\displaystyle 7=l$

$\displaystyle P=2l+ 2w$

We have the perimeter and the width, so we can plug those values into our equation and solve for our unknown. 

$\displaystyle 36=2l+2(8)$

$\displaystyle 36=2l+16$

Subtract $\displaystyle 16$ from both sides

$\displaystyle 36-16=2l+16-16$

$\displaystyle 20=2l$

Divide $\displaystyle 2$ by both sides

$\displaystyle \frac{20}{2}=\frac{2l}{2}$

$\displaystyle 10=l$

$\displaystyle P=2l+ 2w$

We have the perimeter and the width, so we can plug those values into our equation and solve for our unknown. 

$\displaystyle 28=2l+2(5)$

$\displaystyle 28=2l+10$

Subtract $\displaystyle 10$ from both sides

$\displaystyle 28-10=2l+10-10$

$\displaystyle 18=2l$

Divide $\displaystyle 2$ by both sides

$\displaystyle \frac{18}{2}=\frac{2l}{2}$

$\displaystyle 9=l$

$\displaystyle P=2l+ 2w$

We have the perimeter and the width, so we can plug those values into our equation and solve for our unknown. 

$\displaystyle 12=2l+2(2)$

$\displaystyle 12=2l+4$

Subtract $\displaystyle 4$ from both sides

$\displaystyle 12-4=2l+4-4$

$\displaystyle 8=2l$

Divide $\displaystyle 2$ by both sides

$\displaystyle \frac{8}{2}=\frac{2l}{2}$

$\displaystyle 4=l$

$\displaystyle P=2l+ 2w$

We have the perimeter and the width, so we can plug those values into our equation and solve for our unknown. 

$\displaystyle 18=2l+2(2)$

$\displaystyle 18=2l+4$

Subtract $\displaystyle 4$ from both sides

$\displaystyle 18-4=2l+4-4$

$\displaystyle 14=2l$

Divide $\displaystyle 2$ by both sides

$\displaystyle \frac{14}{2}=\frac{2l}{2}$

$\displaystyle 7=l$