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 .
