(n+7)(n-5)-(n+9)(n-7) = n^2-5n+7n-35-(n^2-7n+9n-63) = n^2+2n-35-(n^2+2n-63) = n^2+2n-35-n^2-2n+63 = 28
(n+7)(n-5)-(n+9)(n-7) = n^2-5n+7n-35-(n^2-7n+9n-63) = n^2+2n-35-(n^2+2n-63) = n^2+2n-35-n^2-2n+63 = 28