Distribute 4 over (5a - 2b) to find that 4(5a - 2b) = 20a - 8b.
Remember how exponent notation works? If not, get a refresher here. If we distribute something that has a variable over a quantity in parentheses that also contains that variable, we use exponent notation to keep things tidy. Probably couldn't hurt to also spray it down with a few squirts of Glass Plus.