Velocity of a car traveling in a straight line increases from 0 m/s to 30 m/s in eight seconds what is the average acceleration of the car

Average acceleration is how much the car increases (on average) per second.  So, since it increases by 30 m/s in 8 s, then dividing 30 by 8 will give you an average acceleration of 3.75 m/s^2 (remember that acceleration is ever increasing, so the unit is s^2, not just s)

That's one very very very short-lived unstable isotope of Lithium.