$8x^2 - 3(x - 4) = 12$
$8x^2 - 3x + 12 = 12$
$8x^2 - 3x = 12 - 12$
$8x^2 - 3x = 0$
$x(8x - 3) = 0$
$x_1 = 0 : (8x - 3)$
$x_1 = 0$;
$8x_2 - 3 = 0 : x$
$8x_2 - 3 = 0$
$8x_2 = 3$
$x_2 = \frac{3}{8}$.

$5x^3 - x(2x - 3) = 3x$
$5x^3 - 2x^2 + 3x = 3x$
$5x^3 - 2x^2 + 3x - 3x = 0$
$5x^3 - 2x^2 = 0$
$x^2(5x - 2) = 0$
$x^2 = 0 : (5x - 2)$
$x^2 = 0$
$x_1 = 0$;
$5x_2 - 2 = 0 : x^2$
$5x_2 - 2 = 0$
$5x_2 = 2$
$x_2 = \frac{2}{5}$.

$4x - 0,2x(x + 20) = x^3$
$4x - 0,2x^2 - 4x = x^3$
$-0,2x^2 = x^3$
$-x^3 - 0,2x^2 = 0$
$-x^2(x + 0,2) = 0$
$-x^2 = 0 : (x + 0,2)$
$-x^2 = 0$
$x_1 = 0$;
$x_2 + 0,2 = 0 : (-x^2)$
$x_2 + 0,2 = 0$
$x_2 = -0,2$.

9x(x − 3) + (x − 4)(x − 5) = 20
$9x^2 - 27x + x^2 - 4x - 5x + 20 = 20$
$10x^2 - 36x = 20 - 20$
$2x(5x - 18) = 0$
$2x_1 = 0 : (5x - 18)$
$2x_1 = 0$
$x_1 = 0$;
$5x_2 - 18 = 0 : 2x$
$5x_2 - 18 = 0$
$5x_2 = 18$
$x_2 = \frac{18}{5} = \frac{36}{10} = 3,6$.