How do you solve 50x - 40 x = 40?

$x = 4$.
You can simply the terms with $x$ : $50 x - 40 x = 10 x$. So $50 x - 40 x = 40 \iff 10 x = 40$.
$10 x = 40 \iff x = \frac{40}{10} = 4$