Explanation:
The amplitude is 2. Amplitude means height from the x-axis to the crest/trough.
The period is 2pi. It is from crest to crest (next crest) or trough to trough (next trough).
Note that crest are the highest points of a wave, and that troughs are the lowest points of a wave. (we are talking about transverse waves, but this is more of a physics thing).
Function of graph:
By playing around in a graphing calculator, I got the equation to be
2 (cos (x + pi/2)).
the 2 changes the amplitude, and the + pi/2 shifts the graph by pi/2 to the left.