A 6 kg box slides down a ramp. The angle between the ramp and the level ground is 39°. If the coefficient of kinetic friction is 0.2, what is the acceleration of the box?
a) 9.8 m/s^2
b) 7.5 m/s^2
c) 5.6 m/s^2
d) 3.2 m/s^2