How do i simplify an algebraic expression? ex. 9m-7m+2m

You know how to use addition and subtraction right? There is no difference when it has the same variable. For instance, 4 + 4 = 8, what does 4x + 4x =? 4x + 4x = 8x. You simply added an x at the end, that's all.
9 - 7 = 2
9m - 7m = 2m
2 + 2 = 4
2m + 2m = 4m

For your example, the answer is 4m because of this.
When the variables are different, for instance, 4x + 3y = 5, you would try to get it into a formula structure, but that's something else.