22.5 miles
You would create a proportion based on the given information, then cross multiply and solve for the number of miles 1 gallon of gas can go.
Answer:
See explanation
Step-by-step explanation:
1. Factor the function 

2. The zeros (or the solutions of the equation f(x)=0) are x=3 and x=5. So? points (3,0) and (5,0) are the points on the graph that are the solutions of the equation.
3. Find the vertex:

Thus, the coordinates of the vertex are (4,2) and the axis of symmetry is the vertical line that passes trough the vertex: x=4.