How Do You Calculate the Friction Needed to Stop a Sliding Box?

[katem]

Homework Statement

A 95-kg box is sliding along a level floor with a vi of 15m/s.

a) how large of a constant frictional force is needed to stop to box in a distance of 15m?
b)What is the coefficient of friction between the box and the floor.

Homework Equations

I tried to use vf^2=vi^2+2ad.

The Attempt at a Solution

i used the above equation and i found the acceleration by putting vf as 0 but i am stuck.

 

[ideasrule]

If you can find the acceleration, why can't you find the force needed to produce that acceleration? F=ma.