Using s = ut + 1/2a(t^2),  we can write

100 = 4u  + 8a

=> u + 2a = 25             ------(1)

Also, Since the distance traveled in the next 4 seconds is 140m, the total distance traveled in the 8 seconds is 240 m. So we can write

240 = 8u + 32a

=> u +4a = 30              ------(2)

Solving Eq.1 and 2 we can say,

a = 5/2 m/s/s and u = 20 m/s

Hence the velocity at t=6 sec, is 20 + 5/2*6 = 35m/s.