here is a very brief outline of an approach:

Motion 1:

u = 0

a = 4 m/s^2

Motion 2:

Motion 3:By symmetry with motion 1

a = -4 m/s^2

v = 0

Given:

.... (1)

.... (2)

Use the motion data to get and in terms of and substitute into equation (2).

Then solve equations (1) and (2) simultaneously.