General Sine Function question - need help to find "C"

the question is that i have to fit a general sine model (AsinB(x-C)+D) from some data without using technology

the table is as follows (this is a table about temperatures)

jan feb mar apr may jun july aug sept oct nov dec

15, 16, 14.5, 12, 10, 7.5, 7, 7.5, 8.5, 10.5, 12.5, 14

so far i have managed to get y=5.5sin(pi/6)(x-C)+22.5 but the textbook doesn't quite explain exactly how to find "C" without having a graph already drawn. the correct answer is y=5.5sin(pi/6)(x-10.7)+22.5.

Any help on this asap would be greatly appreciated