Mark M. answered 09/23/19
Retired Math prof with teaching and tutoring experience in trig.
The equation has the form y = Acos(Bx + C) + D
Vertical shift = D = -2
Amplitude = lAL = 2 (Let A = 2)
Period = 2π / lBl = 8π. So, lBl = 1/4. Let B = 1/4
Horizontal shift = -C/B = -4C = 4π. C = -π
y = 2cos(x/4 - π) - 2