What is the whole number value of n?

13^2 + 2 * 13 * n + n^2 = 18^2
169 + 26n + n^2 = 324
26n + n^2 = 324 - 169
26n + n^2 = 155
n^2 + 26n - 155 = 0
(n - 5)(n + 31) = 0

n - 5 = 0
n = 5

n + 31 = 0
n = -31not this one because it is negative

so n = 5 <===