How do I make x the subject of b=x/m(a-x)?

"Subject" is in English. In math you "solve".

b = x/m(a - x) Add spaces to improve readability. The first thing you notice is that your presentation is ambiguous. This could be interpreted:

b = x/(m(a - x))

or

b = (x/m)(a - x)

and no way to tell which. So you just need to clarify the problem.