Answer
f(x) = -9 cos((12(pi) / 7)x) -11
The base equation for a cosine graph is
f(x) = a cos(b(x-c))+d
a = amplitude
b = period
c = phase shift
d = vertical shift
if there's a reflection over the x-axis then there is a negative sign in front of the amplitude
I wrote the equation previously but it got deleted so let me know if you have any problems with this explanation