See Eric Nicholas K's answer to Jn's Junior College 1 H2 Maths Singapore question

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I have long forgotten my JC stuff already, so I cannot guarantee that my answer is correct.

5 years ago

Answer is correct, though you might want to factorise the 7^(2N + 2) - 7^(N+2) into
7^(N+2) (7^N - 1)