A guide on how to set up an equation:
Total cookies sold were: 69
Gabby sold: 35
We don’t know how many Ross sold so his cookies are: B (until calculated)
Equation:
35+B=69
Option C is the answer.
We want to calculate the total amount of cookies and in order to do so, we must add 35 and B to calculate 69.
I can give you a more in depth explanation if you want just leave a comment! (Also no need to give any points). I also tutor, so if you need any math help I can leave my email in the comments.