Need to editing.
theory and so the model should be changed, then the Excel estimation should be changed accordingly; i.e. you need to re-run the model with 2 Variables in the right side, X  & X2(squared).

ay2 = 1+ bx2

It should be

Y = a +bX +CX2(X*X).

Your theory and so the model should be changed, then the Excel estimation should be changed accordingly; i.e. you need to re-run the model with 2 Variables in the right side, X  & X2(squared).