#### Answer

$\mu_k = 0.25$

#### Work Step by Step

Since the safe moves with a constant speed, the acceleration is zero. We can set up a force equation to find the coefficient of kinetic friction. Therefore;
$\sum F = ma$
$385~N+350~N-F_f = 0$
$mg~\mu_k = 385~N+350~N$
$\mu_k = \frac{735~N}{(300~kg)(9.80~m/s^2)}$
$\mu_k = 0.25$