Answer
11 N
Work Step by Step
Coefficient of kinetic friction $\mu_{k}=0.25$
$F_{N}$ is equal but opposite to sum of all the forces downward.
That is, $F_{N}=-(mg+F\sin\theta)$
$=-(3.5\,kg\times-9.8\,m/s^{2}+15\,N\times\sin(-40^{\circ}))=44\,N$
$f_{k}=\mu_{k}F_{N}=0.25\times44\,N=11\,N$
