In this problem we want to find the missing side of a right triangle.
We will be able to use something called the Pythagorean Theorem, which states the sum of the squares of a right triangle is equal the square of the hypotenuse.
Long story short, we have this:
In our problem, we are given

So we can use them directly in the formula:

Simplify the left-hand side:

Finally, we'll need to square root both sides to find the value of c:

The missing side is 13 cm.