12, 15, 16, 17, 18.
Заранее спасибо :)

15) y(xy+1)+x(xy+1)+4 (xy+1) = (xy+1)(x+y+4)

16) 3x (x-y^2)+2 (x-y^2)-y (x-y^2)=(x-y^2)(3x-y+2)

17) x (x-y^2)+1 (x-y^2)-y (x-y^2)= (x-y^2)(x-y+1)

18) a (ax+1)+x (ax+1)+2(ax+1)=(ax+1)(a+x+2)