10ax-5ay+2bx-by Factor first two terms: 10ax-5ay as 5a(2x-y) Factor last two terms: +2bx-by as +b(2x-y) 5a(2x-y)+b(2x-y) Let U = (2x-y) 5aU+bU U(5a+b) Replace U by (2x-y) (2x-y)(5a+b) Edwin