The exercise stated that i have to solve the recurrence using the Recursion-Tree Method. I have already finished the base part, which is $\Theta(n^{\lg3})$
But for the recursive part I'm having troubles with this sum: $cn*\sum_{i=0}^{\lg n - 1} (3/2)^i$
After resolving, I got that $T(n)=cn*n^{\lg3} - 2cn$ but this answer doesn't match $\Theta(n^{\lg 3})$ which I got using the Master Method