Okay. The equation is the way to figure out how much can be put into the suitcase.
What you do is substitute the various weights for w, and see if the equation is true.
So, 42 + w ≤ 50.
If you use the wieght of the books, 10lb, it looks like this: 42 + 10 ≤ 50. If you solve that, the equation isn't true, so you can't put the books into the suitcase.