$f(n):=4^{n}*n$ and $g(n):=6^n$
so we need to show that $f(n) \leq c g(n)$
I tried to find upper and lower bounds:
$\frac{4^{n}*n}{6^n} = \frac{4^{n}*n}{(3/2)^n4^n} = \frac{n}{(3/2)^n} $
$\frac{n}{(2)^n}=\frac{n}{(4/2)^n} \leq \frac{n}{(3/2)^n} \leq \frac{n}{(2/2)^n} = n$
However do not think that my approach is useful so any hint would be helpful.