y= 3 cos(x+pi)-3
since cos(x+pi) = -cos(x),
y = -3cos(x) - 3
Not sure what you want to know about it.
-3cos(x)-3 is 3cos(x+pi)-3 shifted left or right by pi, and then flipped over.
Since cosx has a period of 2pi, you can see why this would result in an identical graph.