To solve for m1 in the equation F=G(m1m2/d2), we must multiply both sides by d2.
That is,
F(d2)=G(m1*m2*d2/d2) which cancels out the d2 in the other side.
Simplifying, we get
F(d2)=G(m1*m2)
Now divide both sides by G*m2, we have
F*d2/G*m2=G(m1*m2/m2*G)
Simplifying to reduce the equation,
m1=F*d2/(G*m2)