This shows you step by step how to solve this problem, so unless you can show us what your daughter has put into the calculator, there's not much we can do to rectify the mistake...
My daughter needs serious help with Linear Programming using Graphing Calculator. She has a problem she has been trying to solve for over a month, I have tried to help her and searched endlessly for help. Hopefully, the link below works. Everything comes up fine except for the very last part. We keep getting either an Error message or 137 instead of 78, as the problem shows. Thanks in advance for any help you can provide.
https://www.dropbox.com/s/xjq0i38dz8...%2011%20PM.jpg