Essentially, you're going to want to reverse the steps on x in order to isolate it from the rest of the variables. To do this, perform inverse operations on x. The first one of these is the multiplication of x and m, so divide both sides by m to cancel m out on the right side. Now we have y/m = x + b. The final thing to do in order to isolate x is to subtract b from both sides. This yields us y/m - b = x. This can be rearranged into x = y/m - b, which should be your answer.
