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Answer:
Step-by-step explanation:
Since it is given that it is quadratic it is of the form f(n)=an^2 + bn+c.
Since we know a few terms we can plug in to get some equations:
f(1)= a +b+c = 7
f(2)=4a+2b+c = 16
f(3)=9a+3b+c = 31
Now you've got yourself a system of three equations which I trust you can solve.
I will say that I'm not sure that this is the most efficient solution(check out the link below which might have a better solve).
Here are some nice explanations with the "real" math notation for a slightly different but related problem:
https://math.stackexchange.com/questions/2345256/how-to-find-the-nth-term-of-quadratic-sequences
Good luck!