How to find average acceleration and force without the time

A car is travelling at 85 km/h and collides with a tree.
The front of the car compresses and driver (mass 90kg) comes to rest after travelling 0.8 m.

What is the average acceleration of the driver during the collision?
& what is the average force the driver is subjected to?

Thanks

first convert the speeds to m/s
85 * 1000 / 3600 = 23.6 m/s

Now there are several ways
V^2 = 2 a s
so a = V^2 / (2s) = 23.6^2 /(2* 0.8) = 348 m/s^2

And of course F = ma = 90 * 348 which equates to 31.3 kN