Answer
$M=\frac{9d^2}{2\sqrt x}$
Work Step by Step
Step 1. Write the general form as $M=\frac{kd^2}{\sqrt x}$ where $k$ is a constant.
Step 2. Use the given condition, $24=\frac{k4^2}{\sqrt 9}$, thus $k=\frac{9}{2}$
Step 3. We have $M=\frac{9d^2}{2\sqrt x}$