How do I find the equation of a cosine function with a maximum of 3, a minimum of -3, and a period of 4?
lets see
y=3cos b(x+c)
c can be any constant becasue you have not mentioned phase shift.
where period = 2pi/b
\(\frac{2\pi}{b}=4\\ \frac{\pi}{b}=2\\ b=\frac{\pi}{2}\)
so it is
\(y=3cos[\frac{\pi }{2}(x+c)]\)
check (here is your graph )
https://www.desmos.com/calculator/nuopbcsgsn