Physics problem to fine average force

[rachels5]

A set of crash tests consists of running a test car moving at a speed of 11 m/s (24.2 m/h) into a solid wall. Strapped securely in an advanced seat belt system, a 57 kg (125.4 lbs) dummy is found to move a distance of 0.81 m from the moment the car touches the wall to the time the car is stopped. Calculate the size of the average force which acts on the dummy during that time.

I do not get this question, i have been working on it for days but i am extremely frustrated

I used the equation
v^2 + u^2 = 2as

where v = final velocity, u = initial velocity, a = acceleration, s = displacement.
so i said
0^2 + 11^2=2(a)(0.81)
It gave me the wrong answer.
but i think that i have to use mass, and my teacher has not shown me a formula in which mass is included

 

[Doc Al]

Show what you've done and where you are stuck.

Hints: You can solve this using energy methods (consider the work done by the wall) or kinematics (find the average acceleration).

 

[rachels5]

can you answer it now .

 

[Doc Al]

I used the equation
v^2 + u^2 = 2as

That should be v^2 = u^2 + 2as.

where v = final velocity, u = initial velocity, a = acceleration, s = displacement.
so i said
0^2 + 11^2=2(a)(0.81)
It gave me the wrong answer.

That will give you the acceleration. Then use Newton's 2nd law to find the net force. (Yes, you'll need the mass.)