=[(3x－2y)²－2(3x－2y)(2x－y)＋(2x－y)²]－6(x－y)＋9
=[(3x－2y)－(2x－y)]²－6(x－y)＋9
=(x－y)²－6(x－y)＋9
=[(x－y)－3]²
=(x－y－3)²