\(\displaystyle P=\frac{k}{\sqrt{q}}\)
Sub in the given p=12 and q=36, then solve for k.
Then, after k is found, it is just a matter of subbing the q=16 into the formula.
Some general thoughts:
y varies directly or y is directly proportional to is y=kx
y varies inversely or y is inversely proportional to is y=k/x