# How do you use the binomial formula to find the coefficient of u^(4)z^(15) in the expansion of (3u+z)^19?

The coefficient of ${a}^{r} {b}^{n - r}$ in the expansion of ${\left(a + b\right)}^{n}$ is $n C n - r$ = (n!)/((n-r)!r!)
Answer is 19C4 X ${3}^{4}$ = ${3}^{4}$(19.18.17.16(/(1.2.3.4) = 313956.