Determine the net force on a book sliding on a table if the book is slowing down.

Answer 1

if the velocity is in the +x direction. the acceleration and therefore the force is in the -x direction.
If that is friction force usually it would be written as
F = m a = - m * g *(friction coefficient)

Answer 2

The net force is going opposite the direction of the book, therefore causing the speed of the book to slow down.