The equation you need here is the can be derived in two lines from the fact that the acceleration is the double derivative of the displacement. And for both vehicles the acceleration is constant (either 6 or 0 ft/s).
where is the velocity of the vehicle at the time . Integrating again gives
where is the displacement of the vehicle at the time . In this problem they are both at the traffic light which we will take as and so can be ignored in this solution.
So the displacement of the car is easy as in this case and , so
And in the case of the truck , and
Now we want the time where the car overtakes the truck. IE displacements are equal therefor
And finally to find what that distance is you can substitute into either the or the equation as they will give the same answer.
So the car will overtake the truck 300 ft from the traffic light.
To make sure you have understood this, try working out how fast the car will be travelling at the point that it overtakes the truck.
Hope this helps.