Answer
See below.
Work Step by Step
If we want to choose $k$ elements out of $n$ disregarding the order, not allowing repetition, we can do this in $_{n}C_k=\frac{n!}{(n-k)!k!}$ ways.
The order doesn't matter here in the set of players, thus we have to use combinations. Thus $_{2}C_{1}(center)_{3}C_{2}(guards)_{12-2-3}C_{2}(forward)=2\cdot3\cdot21=126$
