Solve back help please (illegal value function)

[ptc-2846174]

Hello, I have the function f(x)= x(8-5x)(28+5x)/2(2+x)^2 Interval(0<x<1.6) which I have found the deriviative as f'(x) = 1/2x(8-5x)(28+5x)/(2+x)^2 (is this right?) I found this using simplyfy as when i try to solve symbolically it say no solution. From this I have to solve the equation f'(x)=0 for x to find the maximum of f(x). However when I put this into solve back all I come up with is 'Illegal function value ' (what does that mean??) Can you show me how to type it in to solve back please. Thank you I HAVE to use the solve back facility can't do it any other way because that is what is asked of me in the question.

[RichardJ]

There's no point in posting the question here if you already posted it over on the Collaboratory. This forum is a backwater. Go back to the thread over there (the one where Mark Neil posted a file) and tell us which version of Mathcad you have so someone can convert his worksheet for you. In general, when you ask a question it's also usually best to post your own worksheet. The easier you make it for people to answer your question, the more likely you are to get an answer. Richard