I'm having trouble coming up with an equation to differentiate for this problem

Anybody know?A truck crossing the prairies at a constant speed of 110 km/h gets 8 km/L of gas. Gas costs 0.68$/L. The truck loses 0.10 km/L in fuel efficiency for each km/h increase in speed. Drivers are paid 35$/h in wages and benefits. Fixed costs for running the truck are $15.50/h. If a trip of 450km is planned, what speed will minimize the operating expenses?