m = 1/3rb^2
You can use cross mutilplication here. The general rule is:
If a/b = c/d, then ad = bc
So, m(3rb^2) = 1
Since it's multiplication you can move the brackets whereever you want since they don't reall yhave a purpose except to visually seperate.
So, (m3r)b^2 = 1
Divide...