physics
posted by HELP PLEASE!! .
As a traffic light turns green, a waiting car starts with a constant acceleration of 6.0 m/s^2. At the instant the car begins to accelerate, a truck with a constant velocity of 21 m/s passes in the next lane.
A)How far will the car travel before it overtakes the truck?
B)How fast will the car be travelling when it overtakes the truck?

Write equations for the distance from the traffic light vs time for each car. Measure time from when they leave the light.
X1 = 3 t^2
X2 = 21 t
Set X1 = X2 and solve for t. Take the nonzero solution. Use that t and either equation to get the location. The velocity of the car at that time will be 6.0 * t. I expect it will be 42 m/s, since it will have to have the same average velocity as the truck at that time. 
36.75