This is a right triangle, so we can use the Pythagorean Theorem to find x; the Pythagorean Theorem is a^2 + b^2 = c^2, where c is the hypotenuse. In this case, x is the hypotenuse. Let's set up an equation:
15^2 + 36^2 = c^2
225 + 1296 = c^2
1521 = c^2
39 = c
Answer:
x = 39
