10x + 7 = 8x − 9
10x − 8x = −9 − 7
2x = −16
x = −16 : 2
x = −8

20 − 3x = 2x − 45
−3x − 2x = −45 − 20
−5x = −65
x = −65 : −5 = 13

2,7 + 1,9x = 2x + 1,5
1,9x − 2x = 1,5 − 2,7
−0,1x = −1,2
x = −1,2 : −0,1
x = 12

$\frac{13}{18}x + 13 = \frac{7}{12}x + 8$
$\frac{13}{18}x - \frac{7}{12}x = 8 - 13$
$\frac{26}{36}x - \frac{21}{36}x = 8 - 13$
$\frac{5}{36}x = -5$
$x = -5 : \frac{5}{36}$
$x = -5 * \frac{36}{5}$
x = −36