Acceleration from 10 m/s to 30 m/s in 5.0 s

Assumptions: acceleration is constant during the 5.0 s interval.
Use a = (v_f - v_i)/t.
a = (30 m/s - 10 m/s) / 5.0 s
a = 20 m/s / 5.0 s
a = 4.0 m/s^2
The car's acceleration is 4.0 m/s^2.