Do the grouping $4a^{3}x-4a^{2}b+3bm-3amx=\left(4a^{3}x-4a^{2}b\right)+\left(3bm-3amx\right)$, and factor out $4a^{2}$ in the first and $-3m$ in the second group.
$$4a^{2}\left(ax-b\right)-3m\left(ax-b\right)$$
Factor out common term $ax-b$ by using distributive property.
