Answer
See the detailed answer below.
Work Step by Step
a) See the figures below, and recall that: $F_n\rightarrow$ Normal force. $F_G\rightarrow$ Gravitational force (the weight of the object). $f_k\rightarrow$ Kinetic friction force.
b) The acceleration here is directed down the inclined surface in the direction of the chosen positive $x$-direction since the net force is in the positive $x$-direction.
c) The direction of the velocity is also up the inclined in the negative x-direction since the kinetic friction force acts down the incline.
d) A box is pushed up a ramp by a worker. The ramp is rough and since the box is moving up the ramp, the kinetic friction force acts down the ramp, as you see in the figures below. The box is slowing down.
