Alright so I found the mid line to be 15 and amp to be 3.

The formula I got is f(t) = 3 sin ((pi/10)x) + 15

However according to the back of the book it got;

f(t) = 3 sin ((pi/6)(t - 74) + 15

My question is how did they got that formula from? How did they get a B of pi/6? I am really confused about that part. Everything else on the formula I am ok with. Please help me!