3x(2x − 1) − 6x(7 + x) = 90
$6x^2 - 3x - 42x - 6x^2 = 90$
−45x = 90
x = 90 : (−45)
x = −2

1,5x(3 + 2x) = 3x(x + 1) − 30
$4,5x + 3x^2 = 3x^2 + 3x - 30$
$4,5x + 3x^2 - 3x^2 - 3x = -30$
1,5x = −30
x = −30 : 1,5
x = −20

5x(12x − 7) − 4x(15x − 11) = 30 + 29x
$60x^2 - 35x - 60x^2 + 44x = 30 + 29x$
9x − 29x = 30
−20x = 30
x = 30 : (−20)
x = −1,5

24x − 6x(13x − 9) = −13 − 13x(6x − 1)
$24x - 78x^2 + 54x = -13 - 78x^2 + 13x$
$24x - 78x^2 + 54x + 78x^2 - 13x = -13$
65x = −13
$x = -\frac{13}{65} = -\frac{1}{5} = -0,2$