19) x⋅(y-9)-y⋅(9-y) =

19) Заметим, что (y-9) = -(9-y). Значит,

$$x\cdot(y-9)-y\cdot(9-y) = -x\cdot(9-y)-y\cdot(9-y) = (9-y)(-x-y) = -(9-y)(x+y) = (y-9)(x+y)$$

Ответ: $$(y-9)(x+y)$$