Physics is my weakest subject, but it's worth a shot here. Hopefully the answer is right.
velocity(final) = velocity(initial) + acceleration * time
vf = vi + a * t
We want to bring the chihuahua to a stop in half a second. So, velocity(final) = 0 m/s. Its velocity(initial) = 3.3 m/s.
Solving for a, the acceleration.
0 m/s = 3.3 m/s + (a) * 0.5 s
a = -6.6 m/s^2
Force = mass * acceleration
F = m * a
We know the mass of the chihuahua and we just found the necessary acceleration.
Plug those in.
F = (3.6 kg) * (-6.6 m/s^2)
F = -23.76 N