3. Числа а, х и у связаны формулой у = (14x-a)72. Найдите: а) а, если х = 8, y = 11; б) х, если у = 2, a = 42.

a) Найдем a, если x = 8, y = 11:

$$y = (14x - a) \div 7 - 2$$

$$11 = (14 \cdot 8 - a) \div 7 - 2$$

$$11 = (112 - a) \div 7 - 2$$

$$11 + 2 = (112 - a) \div 7$$

$$13 = (112 - a) \div 7$$

$$13 \cdot 7 = 112 - a$$

$$91 = 112 - a$$

$$a = 112 - 91$$

$$a = 21$$

б) Найдем x, если y = 2, a = 42:

$$y = (14x - a) \div 7 - 2$$

$$2 = (14x - 42) \div 7 - 2$$

$$2 + 2 = (14x - 42) \div 7$$

$$4 = (14x - 42) \div 7$$

$$4 \cdot 7 = 14x - 42$$

$$28 = 14x - 42$$

$$14x = 28 + 42$$

$$14x = 70$$

$$x = 70 \div 14$$

$$x = 5$$

Ответ: а) a = 21; б) x = 5