2011/026) Factor by^2-b^2y-ay^2+a^2y+ab^2-ba^2

rewrite in decreasing power of any one say y and then proceed

by^2-b^2y-ay^2+a^2y+ab^2-ba^2

= by^2-ay^2- b^2y+a^2y+ab^2-ba^2
= y^2(b-a) +y (a^2-b^2) + ab(b-a)
= (b-a) (y^2 + y(a-b)(a+b) + ab)
= (b-a) (y^-y(a+b) + ab)
= (b-a) (y-a)(y-b)

thus it becomes easy