I've been given the following information:

First, I was asked to derive a sine function to model the data, which I got as: Y=3.12sin[pi/6(x-4)]+12.2

Now, I need to get a cosine function to model it. This is where I'm stuck; the sine curve has one maximum, as does the data, so I was easily able to figure out the phase shift. But a cosine curve has two maximums so I'm having a little trouble figuring out a cosine equation.

I decided to check the back of the book and see the answer as y=3.11cos[0.51(m-6.71)]+12.14

At this point, I'm completely lost

Could anyone point me into the right direction as to how I'd be able to derive a cosine function from the above data?