In order to determine the numbers in the parenthesis, we need to find numbers that multiply to the constant (36) and add up to the middle term (13). In order to do that, we list the factors of the constant and look for those that add up to 13.
1, 36
2, 18
3, 12
4, 9
6, 6
As you can see, 4 and 9 are the only pair that satisfy the stipulations. So we place them in a parenthesis with x's and express as the following.
