A football player, starting from rest at the line of scrimmage, accelerates along a straight line for a time of 1.5 s. Then, during a negligible amount of time, he changes the magnitude of his acceleration to a value of 1.4 m/s2. With this acceleration, he continues in the same direction for another 1.2 s, until he reaches a speed of 3.4 m/s. What is the value of his acceleration (assumed to be constant) during the initial 1.5 s period?

Answer 1

Well,this is a math exercise more than physics.
Compute the value of Vf at the end of each period, and use that as the Vi for the next period.
First period:
Vf=a*1.5
Second Period
Vf=a*1.5+1.4*1.2=3.4
solve the last equation for a.

Answer 2

Thanks, it really helped