Answer:
y = 1/2cos(x/2 -5π/4) -1
Explanation:
You want the equation of the shifted and scaled cosine function shown in the graph.
Translation
A point on a function f(x) will be translated (right, up) = (h, k) by the transformation ...
g(x) = f(x -h) +k
Scaling
A function will be vertically expanded by a factor of p and horizontally expanded by a factor of q by the transformation ...
g(x) = p·f(x/q)
Graphed function
The graph shows a cosine function with a peak-to-peak amplitude of 1, which is 1/2 the parent function's amplitude, so p=1/2.
The period is (9π/2 -π/2) = 4π, which is twice the period of the parent cosine function, so q=2.
The first peak of the graphed waveform is at x=5π/2, and the midline of the graphed waveform is y=-1, so we have (h, k) = (5π/2, -1).
Putting these transformations together, we find the equation of the graph to be ...
y = 1/2cos((x - 5π/2)/2) -1 . . . . . . scaling applied before translation
y = 1/2cos(x/2 -5π/4) -1