Algebra problem, need working out as well.

Make m1 the subject

t= m1-m2/1+m1m2

(T = m1-m2 divided by 1+ m1 times m2)

T(1+m1m2) = m1-m2
T+Tm1m2 = m1-m2
T+Tm1m2+m2 = m1
T+m2 = m1-Tm1m2
T+m2 = m1(1-Tm2)
(T+m2)/(1-Tm2) = m1