solve each forumla for the specified variable.
d = cube root 16Mr^2 divide by m
OK then recall that $\displaystyle y=\sqrt[3]{x} \Longleftrightarrow y^3=\sqrt[3]{x}^3=x$. Using this, we get
$\displaystyle d=\sqrt[3]{\frac{16Mr^2}{m}} \Longleftrightarrow d^3=\frac{16Mr^2}{m}$and you're able to solve the last equation for $\displaystyle M$, aren't you ?