So lets fill out what we have first:
Vi or initial velocity = 20 m/s
Acceleration or a = 4 m/s^2
Time for the motion = 10s
Now, using the four main kinematic equations we can deduce that the best kinematic equation to use in these terms is:
Δx = Vi(t) + 0.5at²
Plug all of our information in:
Δx = (20)(10) + (0.5)(4)(100)
Δx = 400 m