Первая дробь:
n(m-3n). n
---------------- = -------
(m-3n)(m+3n). m+3n
приводим к общему знаменателю:
n(n+m)-n^2+3m^2+2mn
------------------------------ =
(n+m)(m+3n)
n^2+nm-n^2+3m^2+2mn
= ---------------------------------=
(n+m)(m+3n)
3m^2+3mn. 3m(m+n)
= ------------------- = --------------- =
(n+m)(m+3n). (n+m)(m+3n)
3m
= ---------
m+3n