In the figure, a 6-kg box is sliding up an incline that makes an angle of theta=30 degree (with respect to the horizontal). As it slides up the incline, a kinetic frictional force fk acts on it. The acceleration of the block has magnitude 8 m/s2. Let the x+ direction point up the plane and the y+ direction be perpendicular to the plane.

Question

In the figure, a 6-kg box is sliding up an incline that makes an angle of theta=30 degree (with respect to the horizontal). As it slides up the incline, a kinetic frictional force fk acts on it. The acceleration of the block has magnitude 8 m/s2. Let the x+ direction point up the plane and the y+ direction be perpendicular to the plane. 
(A) draw free body diagram **I did this
(B) What of the following statements correctly describes the x component ax of acceleration and the y component ay of acceleration as the block slides up the inclined plane? **answer: ax is negative, while ay is zero
(C) What is the normal force acting on the box as it slides up the inclined plane? **answer: FN = 51.96 N
(D) What is the kinetic frictional force acting on the box as it slides up the inclined plane? **answer: 18 N

I do not need the answers. I only need help understanding why the answer is the answer for part (B) and help finding out how the answer was gotten for part (D).