My book says the phase shift is -3 and the period is 12 and (this may be stupid) but I keep wanting to say the period is 36 and now Ive confused myself. Please help!!

53. Periodic Revenue For the past three years, the manager of The Toggery Shop has observed that revenue reaches a high of about $40,000 in December and a low of about $10,000 in June, and that a graph of the revenue looks like a sinusoid. If the months are numbered 1 through 36 with 1 corresponding to January, then what are the period, amplitude, and phase shift for this sinusoid? What is the vertical translation? Write a formula for the curve and find the approximate revenue for April