$$x = \frac{450}{9}$$
$$x = 50$$
$$y = 9 \cdot 7$$
$$y = 63$$
$$9 + a = 23 \cdot 9$$
$$9 + a = 207$$
$$a = 207 - 9$$
$$a = 198$$
$$b - 18 = \frac{504}{72}$$
$$b - 18 = 7$$
$$b = 7 + 18$$
$$b = 25$$
Ответ: 1) x = 50, 2) y = 63, 3) a = 198, 4) b = 25.