$\frac{x}{12} - \frac{x}{8} = \frac{7}{6}$
$\frac{2x}{24} - \frac{3x}{24} = \frac{7}{6}$
$-\frac{1}{24}x = \frac{7}{6}$
$-\frac{1}{24}x = \frac{7}{6}$

$\frac{13x}{21} + \frac{9x}{14} = -1$
$\frac{26x}{42} + \frac{27x}{42} = -1$
$\frac{53}{42}x = -1$
$x = -1 : \frac{53}{42}$
$x = -1 * \frac{42}{53}$
$x = -\frac{42}{53}$

$-\frac{3x}{10} - \frac{7}{15} = \frac{x}{6}$
$-\frac{3x}{10} - \frac{x}{6} = -\frac{7}{15}$
$-\frac{9x}{30} - \frac{5x}{30} = \frac{7}{15}$
$-\frac{14}{30}x = \frac{7}{15}$
$x = -\frac{7}{15} : (-\frac{7}{15})$
x = −1