Three books (x, y, and z) rest on a table. x is on table, while y is stacked upon x, and z is stacked upon y. the weights of the books are mx = 4 n, my = 10 n and mz = 5 n. the net force acting on book y is:

Refer to the diagram shown below.

The free body diagram of book y indicates:
a weight of 5 N from book z, and a reaction of 5 N on book z;
a weight of 10 N acting on book x, and a reaction of 10 N from book x

The net downward force on book y is
5 + 10 - 10 = 5 n.

5 N, downward.