I think you're trying to find when the first derivative is zero. At x=0 and x=-21 the derivative blows up (denominator is zero). If I got the derivative right, I think x=-7 is what you're looking for.
Here is a pretty simple equation that I need to find the critical points for. I know how to find the critical points for an equation, but I'm not able to figure this one out.
I got -21 and 0 for my critical values. Are there any more?