#### Answer

$x=39$

#### Work Step by Step

In order to solve this equation, you need to use The Pythagorean Theorem, which is:
$a^{2}+b^{2}=c^{2}$
Now when we replace the variables with numbers we get:
$36^{2}+15^{2}=x^2$
(In this case, $x$ represents $c$)
When you simplify this equation, you get:
$1296+225=x^{2}$
When we simplify this further by adding the first two numbers, we get:
$1521=x^2$
Finally, we can take the square root of both of these numbers:
$\sqrt x^2=\sqrt1521$
Finally, we get our answer:
$x=39$