1. The problem asks how to find the length of the third side of a right triangle when the lengths of the other two sides are known. 2. The key formula for right triangles is the Pythagorean Theorem, which states: $$a^2 + b^2 = c^2$$ where $a$ and $b$ are the legs (the two sides forming the right angle) and $c$ is the hypotenuse (the side opposite the right angle). 3. This theorem allows you to set up an equation to find the missing side length by plugging in the known side lengths and solving for the unknown side. 4. The other options (sum of side measures, similar triangles, or proportions) are not the standard or direct method for finding the third side in a right triangle when two sides are known. 5. Therefore, the correct phrase is: "the Pythagorean Theorem to set up and solve an equation to find the length of the third side." Final answer: Use the Pythagorean Theorem.