Решим уравнение:
$$\frac{12}{7-x}=x$$
$$12 = x(7-x)$$
$$12 = 7x - x^2$$
$$x^2 - 7x + 12 = 0$$
По теореме Виета:
x1 + x2 = 7
x1 * x2 = 12
x1 = 3, x2 = 4
Проверим:
$$\frac{12}{7-3} = \frac{12}{4} = 3$$
$$\frac{12}{7-4} = \frac{12}{3} = 4$$
Ответ: 3; 4