Since an equilateral triangle has three equal sides, the total perimeter of the triangle is $\displaystyle 9$ units, since each side is $\displaystyle 3$ units long and $\displaystyle 3+3+3=9$.

We know two of the lengths of the triangle's sides: $\displaystyle 5$ and $\displaystyle 3$. We are also told that its third side's length is an average of its other two sides. We can find the average of $\displaystyle 5$ and $\displaystyle 3$ by adding them together and dividing by two: $\displaystyle 5+3=8$ and $\displaystyle 8\div2=4$, so the triangle's third side has a length of $\displaystyle 4$. We can now find the perimeter of the triangle by adding up the lengths of its sides: $\displaystyle 3+4+5=12$, so the perimeter of the triangle is $\displaystyle 12$.

There is no certainty of the type of triangle described in this situation.  This can be an equilateral triangle, isosceles triangle, scalene triangle, or a right triangle.

There is not enough information to determine the perimeter since we only have the dimension of one side.

Write the Pythagorean Theorem.

$\displaystyle a^2+b^2 =c^2$

In an isosceles triangle, the legs are equal:  $\displaystyle a=b$

$\displaystyle a^2+a^2 = 2^2$

Solve for the leg.

$\displaystyle 2a^2 = 4$

$\displaystyle a^2 =2$

$\displaystyle a=\sqrt{2}=b$

The perimeter is the sum of the legs and the hypotenuse.

$\displaystyle a+b+h= \sqrt2+\sqrt2+2=2\sqrt2 + 2$

The perimeter of a triangle is the sum of the three sides.

Add the lengths together to find the perimeter.

$\displaystyle P=8+7+13 = 15+13 = 28$

Because the question asks you to find the PERIMETER of the triangle, you are looking to figure out the distance around the outside of the triangle. You find the perimeter of triangles simply by adding the length of the three sides together. So the correct answer here is $\displaystyle 3 \: cm + 3 \: cm + 4 \: cm$, or $\displaystyle 10 cm$.

Because the question asks you to find the PERIMETER of the triangle, you are looking to figure out the distance around the outside of the triangle. You find the perimeter of triangles simply by adding the length of the three sides together. So the correct answer here is $\displaystyle 50\: cm + 50\: cm + 70\: cm$, or $\displaystyle 170 cm$.

Because the question asks you to find the PERIMETER of the triangle, you are looking to figure out the distance around the outside of the triangle. You find the perimeter of triangles simply by adding the length of the three sides together. Then the correct answer here is $\displaystyle 25 \: cm + 25 \: cm + 30\: cm$, or 80 cm.

When finding the perimeter of any triangle, you will need to add all three sides. In this case, it is important to note that this is the perimeter of an equilateral triangle. An equilateral triangle has $\displaystyle 3$ congruent/equal sides. All the choices add up to $\displaystyle 24$ mm (except the choice that shows multiplication instead of addition), but only one choice shows $\displaystyle 3$ equal sides that add up to      $\displaystyle 24$mm:

$\displaystyle 8 \: mm+8\: mm+8\: mm$

Because the question asks you to find the PERIMETER of the triangle, you are looking to figure out the distance around the outside of the triangle. You find the perimeter of triangles simply by adding the length of the three sides together. So the correct answer here is $\displaystyle 15\: cm + 15 \: cm + 15 \: cm$, or $\displaystyle 45 cm$.