What is the acceleration of a car moving in a

constant velocity of 30 m/s in a straight line?

zero

Explanation:

If the car is moving at a constant velocity then there cannot be any acceleration. Acceleration is any change in velocity over time. If velocity doesn't change, then there is no acceleration.

Acceleration is the change in velocity over the change in time. If velocity is constant or not changing it doesn't accelerate. So 0