Calculus of a Single Variable, Chapter 3, 3.3, Section 3.3, Problem 21

Critical points are where derivation is equal to zero i.e.
f'(x)=0
Therefore, we will calculate function derivative.
f'(x)=-6x^2+6x-12
Hence we need to find solution to the following equation.
-6x^2+6x-12=0
Divide by -6.
x^2-x+2=0
Apply quadratic formula x_(1,2)=(-b pm sqrt(b^2-4ac))/(2a)
x_(1,2)=(1 pm sqrt(1-8))/2=(1 pm i sqrt(7))/2
The above equation has no real roots meaning that the function has no critical points. Hence the function is increasing or decreasing over the whole domain. To check whether it is increasing or decreasing we only need to check sign of the derivative at any point (any point will do because the function is monotone over whole the domain).
f'(0)=-12
Since the derivative is negative the function is decreasing over whole the domain i.e. (-oo,+oo).
Graph of the function is shown on the image below.