The derivitive of $\displaystyle 200(80-x^2)$ is not $\displaystyle -400x(80-x^2)$ ?
what else could it be ?
well u don't need to use the chain rule here.I mean chain rule would have worked if there was soemthing like this
200x(1+800x) u see the difference.In this case the easiest thing to do would be to multiple both the terms with 200 and then differentiate it but then
you cna use teh chain rule as well and it will work because there are 2 x's .idk
hope that helps