How do you factor 2x^2 - 40x?

$2 x \left(x - 20\right)$
Remember that factoring means to "take out" a common factor from each term, namely one from $2 {x}^{2}$ and one from $40 x$.
It happens to be the case that both, $2 {x}^{2}$ and $40 x$ are divisible by $2$, and both share an $x$, which means that we can take out a $2$ as well as an $x$, giving us
$2 {x}^{2} - 40 x \to 2 x \left(x - 20\right)$