Answer
$\mu_k = tan(\theta)$
Work Step by Step
When the skier moves down the slope, the component of $F_g$ directed down the slope is $mg~sin(\theta)$.
If the speed is constant, then the force of kinetic friction $F_f$ opposing the skier's motion must be equal in magnitude to this component of $F_g$ directed down the slope. Therefore,
$F_f = mg ~sin(\theta)$
$mg ~cos(\theta) ~\mu_k = mg ~sin(\theta)$
$\mu_k = \frac{sin(\theta)}{cos(\theta)}$
$\mu_k = tan(\theta)$
