Give an equation for a cosine funtion that has a maximum at (pi/4, 8), followed immediately by another maximum at (9pi/4,8), and has a minimum at value of 4.

This is what I did

Amplitude = (8-4)/2

= 2

Vertical shift = 8-2 =6

So far I got this

y = 2cos (x -? )+6

How do I find the phase shift