720x + (-20x)
Remember
(+)(+) = (+)
(+)(-) = (-)
So + (-20x) will equal - 20x, so
720x + (-20x) = 720x - 20x
Subtract 20 from 720
720 - 20 = 700, so
720x + (-20x) = 720x - 20x = 700x
