Her equation doesn't fit the word problem! It says several 2 pounds books but she put it as if there was one. She should have wrote the equation like this:
3 + 2x = 20 (where x represents the unknown amount of boxes)
and than you can just solve for x to find how many books there are
3 + 2x = 20
2x = 20
x = 10
So there are 10 books!
I hope that helps! Let me know if you have any questions :3
