Can someone answer this 2 correctly please

I know the first one is C.) 4J. I don't know of the answer for the second oneis suppose to be in N/m form? but I got

2,500N/m

C.  

The force of friction = coefficient of friction * normal force.  

Adding the book to the boy increases the normal force and the component of the gravitational force directed down the slide.  This in turn increases the force of friction as can be seen by the relationship from the above equation.  For a stationary object, the force of static friction is always equal to the force applied (in this case, it is the component of the gravitational force directed down the slide).  That means that so long as the boy is not moving and his mass increases, the frictional force is increasing also to balance the increased downward gravitation force directed down the slide.