The Phase Shift is how far the function is shifted horizontally from the usual position.
We have a function,
and we are asked to find its inverse function.
An inverse function essentially gets you the original value that was dropped into a function.
For example,
If I put 5 into 
I will get 24. Now If I take 24 and put it into the inverse function I have to get number 5 back.
The way to determine the inverse function swap the x and the , then solve for ,
Of course the notation demands that the obtained function be called,
Hope this helps :)
C(x)=- 0.3x^2+600x
y=ax^2+bx+c
it shows a upside down parabola equation.
to find maximum value we need to find its vertex(h,k)
as this is a standard quadratic equation we need to see parabolis equation too
y=a(x-h)^2+k
if u see this eq^n K would be the maximum value of Y if x=h.
where h,k are vertex of parabola.
h=-b/2a(derived standard formula)
C(x)=- 0.3x^2+600x
y=ax^2+bx+c
a= -0.3 b=600
h=-600/-0.3
h= 2000
put h in place of x to get K
K= -0.3(2000)^2+600(2000)
K= - 1200000+1200000
K=0
u gets k=0
means C(x)=0
600x= -0.3x^2
600= 0.3x
x=6000/3
x= 2000 units
