The contents of the fuel tank of Sierra's car can be modeled by the function g(x) = -0.04x+15, where z represents the number of miles driven and g(x) represents the number of gallons of gas in the tank. Sierra started her trip with a full tank of gas and traveled 200 miles. Which equation represents the amount of gas that Sierra had left in her car's fuel tank at the end of the trip?
A g(200) = 7
B g(200) = 8
C g(7) = 200
D g(8) = 200