Originally Posted by
Chris L T521 $\displaystyle F=\frac{Gm_1m_2}{d^2}$
First, if you're solving for $\displaystyle m_2$, multiply both sides by the denominator, $\displaystyle d^2$:
$\displaystyle Fd^2=Gm_1m_2$
Can you try to get $\displaystyle m_2$ by itself?