by+by-y-x-ay-ax Представьте многочлены в виде произведения

bx + by - y - x - ay - ax = bx + by - ay - y - x - ax = b(x + y) + (-ay - y) + (-x - ax) = b(x + y) - (ay + y) - (x + ax) = b(x + y) - y(a + 1) - x(1 + a) = b(x + y) - y(a + 1) - x(a + 1) = b(x + y) + (-x(a + 1) - y(a + 1)) = b(x + y) - (x(a + 1) + y(a + 1)) = b(x + y) - (a + 1)(x + y) = (x + y)(b - (a + 1)) = (x + y)(b - a - 1)

Ответ: (x + y)(b - a - 1).