Answer
$m=\displaystyle \frac{H}{S(t_{1}-t_{2})}$
Work Step by Step
The goal is to isolate the wanted variable on one side of the equation.
Multiply with $m$
$ Sm=\displaystyle \frac{H}{(t_{1}-t_{2})}\qquad$ ... divide with S
$m=\displaystyle \frac{H}{S(t_{1}-t_{2})}$
