Answer: y = -cos2x
Step-by-step explanation:
Its a sine wave. Or a cosine wave. Theyre the same depending on how you phase shift it.
I see it as a reflection of the cosine wave over the x axis. this way there are no ugly phase changes. this makes the whole function negative though.
The amplitude can be seen as 1. so there is no vertical stretch.
However, there is a period change. in 2pi, we can see 2 full cycles of this graph, which suggests the period has been shrunk, (by a factor of 2).
so b = 2.
plugging all this information into the cosine function, our equation is going to be:
y = -cos2x
ask questions in the comments if you dont get it :)