Revenue and quadratic formula - for every x increase in price there are y fewer sales
$x$ is not the price increase, $\$5$ is the price increase.
$x$ is just the number of price increases (like, say, number of five-dollar bills customer would have to pay extra) and also the corresponding number of sales decreases (each decrease is $10$ sales).
One price increase brings one sales decrease, so:
$$ y = (80 + 5\cdot 1)(300 - 10\cdot 1). $$ Two price increases bring two sales decreases:
$$ y = (80 + 5\cdot 2)(300 - 10\cdot 2). $$
And so on.