Alright, so solving for the net force is a rather simple and easy step. I want to briefly explain how you determine the net force, but before I do, let's examine the problem together. What has been provided to us? What direction is it going? Well, we can look at the information!
Amanda's Force: 5 N
Jeremiah's Force: 10 N
They're both pushing on the books, but in different directions. Left and right. If Amanda is pushing the books to the right 5 newtons, and Jeremiah is pushing the books to the left with 10 newtons, that means the net force is 5. The book is being pushed to the left! We can say that because Jeremiah's force is much larger than his sisters.
We can determine the net force from the following;
Net Force → 10 N - 5 N = 5 N
In conclusion, your answer should be a total of five (5) newtons