what did I do wrong with this quadratic function graph question?

[AhmedAlmosuli]

 

[MarkFL]

I agree with you that f(6) = 7.

 

[Dr.Peterson]

View attachment 35169

Assuming you were told that f is quadratic, the answer of 7 is an immediate implication of symmetry. Reflect the y-intercept over the line of symmetry.

So clearly the given answer is wrong.