kagesuki

Mathway is a pretty great website, but it's not for all types of problems.

I haven't gotten up to that part in my physics classes yet, but you should try using variables to sub for the values you don't know. 

Right now, what you have is this:

initial velocity: 6.0 m/s

final velocity: 0 m/s

Coefficiant of kinetic friction: 0.100

Draw a force diagram. Since it looks like you have more information on velocities, you might be able to find acceleration or something like that, use a few variables and more equations, and solve for the answer.

If the question only gives you two pieces of information, you can try using variables to sub for the other variables you'll need to solve for the question. Most likely, the variables will cancel out, leaving you with the answer.