x^2-a(3x-2a+b)-b^2
=x^2-3ax+2a^2-ab-b^2
=(x^2-3ax+9/4a^2)-(1/4a^2+ab+b^2)
=(x-3/2a)^2-(1/2a+b)^2
=(x-3/2a+1/2a+b)(x-3/2a-1/2a-b)
=(x-a+b)(x-2a-b)