Two masses M$1$ and M$2$ are $12$ Kg and $4$ Kg respectively. What is the net downward acceleration?

$4\mathrm{.}9\frac{m}{s^2}$

$9\mathrm{.}8\frac{m}{s^2}$

$19\mathrm{.}6\frac{m}{s^2}$

$0$