Answer:
(x+y)(a+b)-(y+z)(a+b)
first we solve half
(x+y)(a+b)
(xa+xb+ya+yb)
(y+z)(a+b)
(ya+yb+za+zb)
(xa+xb+ya+yb)-(ya+yb+za+zb)
xa+xb+ya+yb-ya-yb-za-zb
which needs to cut
+ya-ya+yb-yb
now left
xa+xb-za-zb
x (a+b)-z (a+b)
(x-z)(a+b)2
