40621

The function is 5cos(x-Pi/4)

You can solve these types of problems by firstly inspecting the maximum and minimum values of the graph. Since the maximum and minimum values of a y=cos(x) graph are 1 and -1, the function must involve y=5cos(x). The function is the graph of y=5cos(x) shifted to the right by Pi/4. Therefore you subtract Pi/4 by the x-coordinate.

Hence:

5cos(x-Pi/4)