Answer
$\mu_k = 0.12$
Work Step by Step
Since the sled is moving at a constant speed, the net force on the sled is zero. Therefore, the force of kinetic friction is equal in magnitude to the horizontal component of the applied force.
$F_f = (75~N)~cos(30^{\circ})$
$F_N~\mu_k = (75~N)~cos(30^{\circ})$
$[mg-(75~N)~sin(30^{\circ})]~\mu_k = (75~N)~cos(30^{\circ})$
$\mu_k = \frac{(75~N)~cos(30^{\circ})}{mg-(75~N)~sin(30^{\circ})}$
$\mu_k = \frac{(75~N)~cos(30^{\circ})}{(60~kg)(9.80~m/s^2)-(75~N)~sin(30^{\circ})}$
$\mu_k = 0.12$
