a) 2a(x+y)+y+x = 2a(x+y)+(x+y)
= (2a+1)(x+y)
b) 2(x-y)^2-x+y = \(2\left(x-y\right)^2-\left(x-y\right)=\left(2x-2y-1\right)\left(x-y\right)\)
c) x^2-xy-3x+3y = x(x-y) - 3(x-y)
= (x-3)(x-y)
d) ax^2+6x^2-6x-ax+a+b=\(ax\left(x-1\right)+6x\left(x-1\right)+\left(a+b\right)=x\left(a+b\right)\left(x-1\right)+\left(a+b\right)=\left(a+b\right)\left(x^2-x-1\right)\)