How to calculate the turning point / math question

[happygolucky2012]

How do I find the maximum turning point in Eviews If I have a quadratic model in the form of



Substituted Coefficients:
=====================
DY = 3.82e-005 + 4.24*DX - 5e-011*DGDPCAPITASQ - 0.35*RESID_Y(-1)

Where
DY is the first difference of Y
DX is the first difference of X
DXSQ is the first difference of x^2
resid_Y is the residuals from the previous period of DY c DX DX^2

I haven't taken a math course in a long time so I pulled this from google

f(x) = x² − 6x + 5.
f '(x) = 2x − 6 = 0 implies x = 3.
f(x) = x² − 6x + 5
f(3) = 3² − 6· 3 + 5
= −4
which will get me a minimum value.

How do I solve this in Eviews or by hand with the residual term when the residual term is a lag value of the previous period? My head hurts.

Thank you,

[happygolucky2012]

hmmm

the residual term is a number so it cancels out when you take the first derivative?