It's impossible to tell what you did wrong if you don't show exactly what you did. HOW did you get "(V^2C/L^2)" as the acceleration?
The truck is traveling at constant velocity V so if we let "t" be the time until the truck reaches the passing lane, Vt= L (and so t= L/V).
With constant acceleration, a, the car travels distance (1/2)at^2+ Vt in time t and, because it is distance C behind the truck, must travel distance L+ C in the same time to be beside the truck: (1/2)at^2+ Vt= L+ C= Vt+ C. We can cancel the "Vt" terms to get (1/2)at^2= C. Put t= L/V into that. I don't see any need for Calculus here.