a, x(x -2) - (x+5)(x - 1)
=x^2 - 2x - x^2 + 5x - x +5
= x^2 - x^2 -2x - 5x - x +5
= 0 + ( -2- 5 +1 )x +5
=-6x + 5
b,2(x - y) ( x + y ) + (x +y )^2 + (x-y) ^2
=2(x^2−y^2)+(x^2+2xy+y^2)+(x^2−2xy+y^2)
=2x^2−2y^2+x^2+2xy+y^2+x^2−2xy+y^2
=(2x^2−2y^2+x^2+y^2+x^2+y^2)+(2xy−2xy)
=0