That's an arithmetic series.
We substitute k+1 for n in the theorem statement to get the goal of the induction step:
2 + 4 + 6 + ... + 2k + 2(k+1) = (k+1)(k+2)
The step that gets us to that goal is the last step, where we take the statement for k and add 2(k+1) to both sides.
Last choice
