If the function:
is typed correctly, then the derivative is equal to the derivative of:
We need to use the product rule here:
Factor the derivative:
Solving for the critical number you get:
so clearly something went wrong in my rushed reply here, because this does not agree with your book's answer. or... the original equation could be different and I misinterpreted the parentheses. I hope the procedure helps though.
Because you have a closed interval for the domain. You should compare the population values at the end points to the ones at the critical numbers.