3x = 6
$x = \frac{6}{3}$
x = 2

$\frac{1}{3}x = -5$
$x = -5 : \frac{1}{3}$
x = −5 * 3
x = −15

−2x = 12
$x = -\frac{12}{2}$
x = −6

$\frac{3}{7}x = 9$
$x = 9 : \frac{3}{7}$
$x = 9 * \frac{7}{3}$
x = 3 * 7
x = 21