Correct Answer - 24 J
`FBD` of the block,
`f_(L) = 6N, F_("pseudo") = 4N`
:. `f = 4N`
acc. Of the block with respect to observes
`= 2-5 = -3 m//s^(2)`
`:.` Displacement of the block w.r.t. observes
`= (1)/(2) xx -3 xx 4 = -6 m`
`:.` work done by friction w.r.t. observers
`= -24 "Joule"`.