As it is given here
initial speed is
v_i = 6 m/s
After 4 seconds the final speed is
v_f = 14 m/s
so here we can use the formula of acceleration using kinematics
a = \frac{v_f - v_i}{t}
a = \frac{14 - 6}{4}
a = 2 m/s^2
so here it will accelerate at 2 m/s^2 rate.
