Answer
$r=12/5=2.4$
Work Step by Step
"r varies directly as m squared and inversely as s" can be written as:
$r=\frac{km^2}{s}$, with $k$ is a constant.
Because we are given with all 3 values $r, m, s$ in a set, we are able to find the constant $k$.
$12=\frac{k*6^2}{4}$
$36k=12*4=48$
$k=4/3$
Then, we use the $k$ value just found to solve for r with another values of $m$ and $x$.
$r=\frac{4/3*6^2}{20}=\frac{4}{3*20}*36=36/15=12/5$