1. **State the problem:** We need to find the length of the hypotenuse of a right triangle with legs 36 and 15. 2. **Recall the Pythagorean theorem:** For a right triangle with legs $a$ and $b$, and hypotenuse $c$, the relationship is $$a^2 + b^2 = c^2$$ 3. **Identify the sides:** Here, the legs are $a=36$ and $b=15$. The hypotenuse $c$ is unknown. 4. **Substitute the known values:** $$36^2 + 15^2 = c^2$$ 5. **Calculate the squares:** $$1296 + 225 = c^2$$ 6. **Add the values:** $$1521 = c^2$$ 7. **Find the hypotenuse by taking the square root:** $$c = \sqrt{1521}$$ 8. **Calculate the square root:** $$c = 39$$ **Answer:** The length of the hypotenuse is 39. **Regarding the first step:** The correct first step is D: Substitute the side lengths for $a$ and $b$ in the equation $a^2 + b^2 = c^2$.