I'm going to try and make this as simple as possible to explain.
-4(5 - 2x) = 8
First of all, we have to distribute the outside term (-4) throughout the inside terms on the parenthesis (5 - 2x). Basically what we're doing is multiplying -4 by each term.
-20 + 8x = 8
Why +8x? It's because a negative times a negative is a positive.
Now, since we want x all by itself, we have to move the constant, -20, to the other side by adding it on both sides.
8x = 28
In order to have x all by itself, you have to divide both sides by 8.
Once you divide 28 by 8, you'll get the quotient and that, ultimately, will be x.
Hope this made it simple and understandable.