Magnitude of work done by friction is Ffd = mgμd = Work done by friction against the cart's motion. You can solve for mu, the coefficient of friction as you know m, d, and Work.
The work of friction is negative.
The question about the "law of conservation" is not clear. Conservation of energy is always true (except nuclear processes). Is energy conserved for the block? NO The 1st law for the block is
ΔEk = -Wf (if we take the W of dissipative friction to be its magnitude (it always goes against kinetic energy)
Energy is conserved globally in the sense that the kinetic energy transitions to thermal energy of the environment and the block by the mechanism of friction.
