x^2-xy+ax-y+a-1 =(x^2-1)-(xy+y)+(ax+a)=(x-1)(x+1)-y(x+1)+a(x+1)=(x-1-y+a)(x+1)
x^2-xy+ax-y+a-1 =(x^2-1)-(xy+y)+(ax+a) =(x-1)(x+1)-y(x+1)+a(x+1) =(x-1-y+a)(x+1)