Answer
$\mu_k = 0.372$
Work Step by Step
We can find $\mu_k$:
$2Mg-Mg-2Mg~\mu_k = (M+2M+2M)~a$
$Mg-2Mg~\mu_k = 5M~a$
$g-2g~\mu_k = 5a$
$2g~\mu_k = g-5a$
$\mu_k = \frac{g-5a}{2g}$
$\mu_k = \frac{(9.8~m/s^2)-(5)(0.500~m/s^2)}{(2)(9.8~m/s^2)}$
$\mu_k = 0.372$
