m^2-4n^2 m^2-4n^2
--------------- : ------------------ =
mn 3n
(m-2n)(m+2n) (m-2n)(m+2n)
= ---------------------- : ---------------------- =
mn 3n
(m-2n)(m+2n) * 3n
= ----------------------------- =
(m-2n)(m+2n) * mn
3n 3
= ------- = ------
mn m
3
ОТВ: -----
m