Alright, so for every addition or subtraction (without the whole thing being in parenthesis and multiplied by something) sign, there is 1 term to the right of it and 1 term to the left of it. However, we have to make sure not to overlap - for example, m is only 1 term - it can't be counted as 2 since it's to the right of a - and to the left of a +. Our terms are therefore 2m²n, 2mn², m, 3n, and 9