Highway saftey engineers build soft barriers so that cars hitting them will slow

Question

Highway saftey engineers build soft barriers so that cars hitting them will slow down a a safe rate. A person wearing a saftey belt can withstand as acceleration os 300 m/s^2.
How thick should barriers be to safely stop a car that hits the barrier at 115km/h?
Answer in units of M.

I already converted the the 15km/h into 31.944 m/s.
I then took the 31.944=300t getting t= .10648sec.
I then pluged all my variables tni this equation:
x(t)=x=vt=1/2at^2 ---> x(t)= 0+31.944(.10648)+1/2(300)(.10648^2)
I get the answer 5.102095m
I dont think this answer is right. What am I doing wrong?

Explanation / Answer

115 km/hour to m/s = 31.94 m/s vf^2 = vi^2 + 2*a*x vf = 0 vi = 31.94 a = -300

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