What's the force if a car travelling at 40 km/h comes to a stop in 0.08 seconds

The force on the seatbelt,...Assume the mass of the occupant is 70kg.

40 km/h = 11.1 m/s

First you need to find the deceleration needed to stop the car in 0.08 s. Then you can use this value to find the force against the seat-belt.

Vf =Vi + at
0 m/s = 11.1 m/s + a(0.08 s)
a = -138.75 m/s^2

F = ma
F = (70 kg)(138.75 m/s^2) = 9712.5 N