There isn't enough information; however, let's say you were to find the missing side length of a rectangle. The perimeter is 52 and the two given side lengths are both 16. You would set up an equation to find the missing side lengths:
52 = 16 + 16 + ? + ?
52 = 32 + ? + ?
20 = ? + ?
Since this is a rectangle, the two other sides have to be equal. Therefore, you would divide the number by 2.
10 = ?