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

(x − 6)(x + 9) = 0
(x − 6) = 0 : (x + 9)
$x_1 - 6 = 0$
$x_1 = 6$;
(x + 9) = 0 : (x − 6)
$x_2 + 9 = 0$
$x_2 = -9$.

(3x + 5)(10 − 0,4x) = 0
(3x + 5) = 0 : (10 − 0,4x)
$3x_1 + 5 = 0$
$3x_1 = -5$
$x_1 = -\frac{5}{3}$
$x_1 = -1\frac{2}{3}$;
(10 − 0,4x) = 0 : (3x + 5)
$10 - 0,4x_2 = 0$
$-0,4x_2 = -10$
$x_2 = -10 : (-0,4)$
$x_2 = 25$.