Answer: Choice A
What we do is simply replace every n with k+1. So (3n)^2 turns into (3(k+1))^2
We see that only choice A has the correct term mentioned on the left hand side, so this must be the answer.
The right hand side is treated the same way. We plug in n = k+1. Your teacher did a bit of algebraic manipulation to get what is shown for choice A.