simplify by removing factors of 1
n^2-9/(n+3)^2
n+3 is a factor of both numerator and denominator.
You should recognize that
n^2-9 = (n-3)(n+3)
Cancel out the n+3 terms.
n^2-9/(n+3)^2
You should recognize that
n^2-9 = (n-3)(n+3)
Cancel out the n+3 terms.