There are three phases for the movement:

- acceleration phase (when it accelerates from 0 m/s to 24 m/s at a rate of 10 m/s^2)

- traveling phase (three hours traveling at 24 m/s)

- deceleration phase (when it decelerates from 24 m/s to 0/ms in 10 seconds)

Both questions can be answered by using the physics formulas for uniform motions.

As for the average speed of the car for the whole journey, you should use the weighted mean of the three phases.... like~

((averagespd acceleration phase)*(time taken on acceleration phase) + (aspd traveling)*(time traveling) + (aspd deceleration)*(time deceleration))/(totaltime)