Question

I need help with :

Answer 1

I think one popular definition of acceleration might help you here:

Acceleration = (change in speed) / (time for the change)

Multiply each side of this equation by (time for the change) :

Change in speed = (acceleration) x (time for the change)

The problem gives you the acceleration and the time, so

Change in speed = (0.73 m/s²) x (7.1 s)  =  5.183 m/s

Before the acceleration, the car's speed was 7.6 m/s . 
Then the car accelerated for a while. 
During the acceleration, its speed grew by 5.183 m/s.
The final speed is  (7.6 + 5.183) =  12.783 m/s  .