Final speed from rest with a = 3.0 m/s^2 for 4.0 s

Assumptions: acceleration is constant and the object starts from rest.
Use v_f = v_i + at.
v_f = 0 m/s + (3.0 m/s^2)(4.0 s)
v_f = 12 m/s
The final speed is 12 m/s.