Use inverse operation.
[tex] \frac{5}{7} m = 15[/tex]
First get rid of the fraction. The fraction is multiplying with x. So the opposite of multiplying is division.
Divide 5/7 with itself and do that with the other side.
[tex] \frac{5}{7} m \times \frac{7}{5} = \frac{15}{1} \times \frac{7}{5} \\ m = 7 \times 3 \\ m = 21[/tex]
