Steps + Answer:
(m/(n(m-n))) + (n/(m(n-m)))
Take mn(m-n)(n-m) as a common denominator
(m(m(n-m)) + n(n(m-n))) / (mn(m-n) (n-m))
= (m(mn-m^2) + n(nm-n^2)) / ((m^2n-mn^2)(n-m))
= (m^2n-m^3+n^2m-n^3) / (m^2n^2-m^3n-mn^3+m^2n^2)
= (m^2n-m^3+n^2m-n^3) / (2m^2n^2-m^3n-mn^3)
This is the final answer
Hope this helps :)