What is the relationship between 12 and -12

Suzanne began a trip with 18 1/2 gallons of gas in the gas tank of her car if she hughes 17 3/4 gallons on the trip how many gallons does she have left once she finish the trip

In which direction does the left side of the graph of this function point? a(x) = 3x - x2 + 4x - 2

If the tip varies directly with the number of guest which equation represents between the tip,t, and the number of guest,g?