Let x=number of cars he sells.
$750 is his base salary.
If he sells no cars, he gets paid $750.
If he sells one car, he gets paid $750+300.
If he sells two cars, he gets paid $750+300×2.
If he sells three cars, he gets paid $750+300×3.
If he sells x cars, he gets paid $750+300×x.
Using the income function above, you can complete the inequality required.