How do you calculate work done by friction?
This is an interesting question!
Friction is a NON-conservative force. This implies that the work done depends upon the trajectory along which the force operates.
To make an example, if you imagine changing disposition of your furniture and push a sofa across the room, depending upon the chosen trajectory you'll have a different amount of work from friction.
Along the first trajectory you have to do a work of
Hope it helps!