i dont understand how i can get one answer out of the graph with so many points. can someone please solve this problem with the steps if possible. i looked in every page in my algebra book and looked online for about an hour and i just cant figure it out. someone please help

i need help with #15 as well. i tried just plugging in numbers and 4 but i couldnt get a pattern or one answer. i wish i had similar problems in my book so i didnt have to bug you guys, sorry.