x^2-(m^2+n^2)x+mn(m^2-n^2)
=x^2-(m^2+n^2)x+mn(m+n)(m-n)
=x^2-(m^2+n^2)x+n(m+n)*m(m-n)
=(x-n(m+n))(x-m(m-n))
=(x-mn-n^2)(x+mn-m^2)