3(m+n)X的平方+6(m+n)XY+3(m+n)Y的平方
=3（m+n)(x^2+2xy+y^2)
=3(m+n)(x+y)^2
(3x+2y)的平方-(2x+3y)的平方
=(3x+2y+2x+3y)(3x+2y-2x-3y)
=5(x+y)(x-y)