原式=2[4(m-n)^2-9(m-2n)^2]
=2[(2m-2n)^2-(3m-6n)^2]
=2(2m-2n-3m+6n)(2m-2n+3m-6n)
=2(-m+4n)(5m-8n)
=-2(m-4n)(5m-8n)