Write the equation of a sine or cosine function to describe

Write the equation of a sine or cosine function to describe the graph.

Amplitude ,A = 4

period = 6pi

General sine equation : y = Asin(Bx - C)

B = 2pi/period = 2pi/6pi = 1/3

y = 4sin(x/3 - C)

at x =15pi/4 ; y = 4 ; 4 = 4sin(15pi/12 - C)

pi/2 = 15pi/12 - C ; C = 15pi/12 - pi/2 = 9pi/12 = 3pi/4

y = 4sin(x/3 - 3pi/4)