(2a-b+c)^2-(3a+2b-c)^2
=[(2a-b+c)+(3a+2b-c)][(2a-b+c)-(3a+2b-c)]
=[2a-b+c+3a+2b-c][2a-b+c-3a-2b+c]
=(5a+b)(-a-3b+2c)