# How does $3^n-1+2 \cdot 3^n$ evaluate to $3^{n+1}-1$?

Can anybody here help me with this simple problem? I've been thinking about this for half an hour and I am not able to come to a solution.

How does $3^n-1+2 \cdot 3^n$ evaluate to $3^{n+1}-1$?

Note that $$3^{n+1} = 3 \cdot 3^n = (1+2)3^n = 3^n+2\cdot 3^n$$
$3^n -1 + 2\cdot3^n$ = $3\cdot3^n -1$
= $3^{n+1}-1$
Hint: $(1+2)3^n-1 \implies 3^{n-1}-1$. Grouping terms.