KVL On A Circuit With 2 Voltage Sources
The sign of the source is drawn incorrectly or the polarity of how \$V_x\$ is defined should be reversed.

And most likely the equation is correct.

$$ Vx + 3Vx + 6i - 20 = 0 $$

together with the relation
$$ V_x=3\cdot i$$

yields

$$ 18i - 20 = 0 $$

If it were
$$ Vx - 3Vx + 6i - 20 = 0 $$
it would yield
$$ -20=0$$