Use proof by mathematical induction to prove explicit formula for recursive sequence is correct
Try using J.G.'s hint and then you can check your answer with the one below.
$P_k = 2 · 3^{k+1} - 14$
Therefore
$P_{k+1} = (2·3^{k+1} - 14)+4.3^{k+1}$
Add the $2·3^{k+1}$ and $4·3^{k+1}$ to get $6·3^{k+1}$
Then
$P_{k+1} = 6·3^{k+1} - 14=2·3^{k+2} - 14$
and you have the correct result.