5:$2.50
9:?
divide 5 by both sides cause you wanna see how much is 1 liter so you can multiply that with 9.
5 divided by 5 is 1.
$2.50 divided by 5 is $0.50
1:$0.50
each liter costs $0.50
Multiply $0.50 with 9 and that is $4.50
the answer is $4.50
The answer to this question is D
Your equation

is in the form

The vertex only relies on "a" and "b" though, so that +9 doesn't really matter in this case. The vertex of a parabola is located where

So your x-coordinate is -1. You need to find f(-1) to find your y-coordinate, and then you list it in the form (-1, y).