Transforming a cosine graph.

I am supposed to write an equation for a graph that shows the starting point at (0,-2) max at (3,0) next min at (just over 6, -2)

and it continues.

18c

http://i166.photobucket.com/albums/u...c/workk004.jpg

So it says write a sine/cosine equation.

I wrote a sine, but the answer shows a cosine, I know it can be either but my question is

for my a value I get 1

my d value is -1

and my x = 3 because the first max is at 3.

it gives y=-cosx-1 ?

so how did they get that?

I have a =+ but If you have a -a does your max become a min?