This is what is called a literal equation. Solve it for m just like you would solve any equation for x. Isolate the term with the variable you're looking for. Do this by subtracting 4cm from both sides to give you g - 4cm = -3m. Now we need to divide by -3 to get m alone: 
. Or we could lose the negative on the 3 in the denominator like this: 
and 
. There you go! Any one of those last three would work. It just depends upon how your teacher wants you to express your answer.