$(x - 4)(x + 3) = 0$
$x_1 - 4 = 0$
$x_1 = 4$;
$x_2 + 3 = 0$
$x_2 = -3$.

$x^2 - 81 = 0$
$(x - 9)(x + 9) = 0$
$x_1 - 9 = 0$
$x_1 = 9$;
$x_2 + 9 = 0$
$x_2 = -9$.

$7x^2 + 21x = 0$
$7x(x + 3) = 0$
$7x_1 = 0$
$x_1 = 0$;
$x_2 + 3 = 0$
$x_2 = -3$.

$9x^2 - 6x + 1 = 0$
$(3x - 1)^2 = 0$
3x − 1 = 0
3x = 1
$x = \frac{1}{3}$

$x(x + 7)(3x - 2) = 0$
$x_1 = 0$;
$x_2 + 7 = 0$
$x_2 = -7$;
$3x_3 - 2 = 0$
$3x_3 = 2$
$x_3 = \frac{2}{3}$.

$12x^3 - 2x^2 = 0$
$2x^2(6x - 1) = 0$
$2x_1^2 = 0$
$x_1 = 0$;
$6x_2 - 1 = 0$
$6x_2 = 1$
$x_2 = \frac{1}{6}$.