If I understand what you mean, you want to combine the two terms into one fraction.

2m +(m+n)/(m-n)

= [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.