Use this kinematics equation:
Vf = Vi + aΔt
Vf is the final velocity, Vi is the initial velocity, a is the acceleration, and Δt is the elapsed time.
Assign northward velocity/acceleration as positive and southward as negative. Given values:
Vi = 100km/h = 27.8m/s
a = -4m/s²
Δt = 6s
Plug in the values and solve for Vf:
Vf = 27.8 - 4(6)
Vf = 3.8m/s north