You have to find when f '(x) is negative. You don't seem to have differentiated yet??
I am to determine the interval on which f(x)=12x-x^3 is decreasing. I am coming up with x(12-x^2). x plus or minus =2square root 3. Solution being (-infinity, -2 square root 3). I have no other way to determine if I am doing this right. Can someone please set me straight?