Given ${\displaystyle \underset{4}{\overset{9}{}}}f(x)dx=16$ and ${\displaystyle \underset{4}{\overset{9}{}}}g(x)dx=-4,$ evaluate the following.

$($a$){\displaystyle \underset{4}{\overset{9}{}}}[f(x)+g(x)]dx$

$$

$($b$){\displaystyle \underset{4}{\overset{9}{}}}[g(x)-f(x)]dx$

$$

$($c$){\displaystyle \underset{4}{\overset{9}{}}}2g(x)dx$

$($d$){\displaystyle \underset{4}{\overset{9}{}}}3f(x)dx$