5x(3x − 2) − 15x(4 + x) = 140
$5x * 3x - 5x * 2 - 15x * 4 - 15x * x = 140$
$15x^2 - 10x - 60x - 15x^2 = 140$
−10x − 60x = 140
−70x = 140
x = 140 : (−70)
x = −2

1,2x(4 + 5x) = 3x(2x + 1) − 9
1,2x * 4 + 1,2x * 5x = 3x * 2x + 3x * 1 − 9
$4,8x + 6x^2 = 6x^2 + 3x - 9$
$4,8x + 6x^2 - 6x^2 - 3x = -9$
1,8x = −9
$x = -\frac{9}{1,8} = -\frac{90}{18}$
x = −5

6x(7x − 8) − 2x(21x − 6) = 3 − 30x
6x * 7x − 6x * 8 − 2x * 21x + 2x * 6 = 3 − 30x
$42x^2 - 48x - 42x^2 + 12x = 3 - 30x$
−48x + 30x + 12x = 3
−6x = 3
$x = -\frac{3}{6}$
$x = -\frac{1}{2}$

12x − 3x(6x − 9) = 9x(4 − 2x) + 3x
12x − 3x * 6x + 3x * 9 = 9x * 4 − 9x * 2x + 3x
$12x - 18x^2 + 27x = 36x - 18x^2 + 3x$
$12x - 18x^2 + 27x - 36x + 18x^2 - 3x = 0$
0 = 0, корнем уравнения является любое число.

$7x^2 - x(7x - 5) - 2(2,5x + 1) - 3 = 0$
$7x^2 - x * 7x + x * 5 - 2 * 2,5 - 2 * 1 - 3 = 0$
$7x^2 - 7x^2 + 5x - 5x - 2 - 3 = 0$
0 − 5 = 0
0 ≠ 5, уравнение не имеет корней.

$8(x^2 - 4) - 4x(3,5x - 7) = 20x - 6x^2$
$8 * x^2 - 8 * 4 - 4x * 3,5x + 4x * 7 = 20x - 6x^2$
$8x^2 - 32 - 14x^2 + 28x = 20x - 6x^2$
$8x^2 - 14x^2 + 28x - 20x + 6x^2 = 32$
28x − 20x = 32
8x = 32
x = 32 : 8
x = 4