If I understand what you mean, you want to combine the two terms into one fraction.
= [2m(m-n) +(m+n)] / (m-n)
= [2m^2 -2mn +m +n] / (m-n)
That is it.
Or, you meant:
2m(m-n) / (m-n), and (m+n) / (m-n)
Those are two fractions having a least common denominator.