A body travels 100m in the first 2seconds and 104m in the next 4seconds.How far will it move in the next 4seconds if the acceleration is uniform?

Using the formula D = v_o t+ \frac 1 2 a t^2 you have two equations in two unknowns:

1) D_1 = 100 = v_o (4) + \frac 1 2 a (4^2)

2) D_2 = D_1 + 100 = 204 = v_o (8) + \frac 1 2 a (8^2)

Solve for v_o and a, then find the distance travelled in 12 seconds:

3) D_3 = v_o (12) + \frac 1 2 a (12^2)

The distance travelled between 8 and 12 seconds is then D_3 - D_2.