2x + x(3 − (x + 1)) = x(2 − x) + 12
2x + x(3 − x − 1) = x(2 − x) + 12
$2x + 3x - x^2 - x = 2x - x^2 + 12$
2x = 12
x = 6

$x^2(5x + 3) - 6x(x^2 - 4) = 3x(8 + x)$
$5x^3 + 3x^2 - 6x^3 + 24x = 24x + 3x^2$
$-x^3 = 0$
x = 0

x(12 − x) − 5 = 4x − x(10 − (3 − x))
$12x - x^2 - 5 = 4x - x(10 - 3 + x)$
$12x - x^2 - 5 = 4x - 10x + 3x - x^2$
12x + 10x − 3x − 4x = 5
15x = 5
$x = \frac{5}{15} = \frac{1}{3}$

x(4x − 11) − 7x(x − 1) = −2x(x + 2) + 1
$4x^2 - 11x - 7x^2 + 7x = -2x^2 - 4x + 1$
$4x^2 - 7x^2 + 2x^2 - 11x + 7x + 4x = 1$
$-x^2 = 1$
нет решений