The product rule is right. What are you getting when you try to apply the product rule?
Show that the function y=x³(1-x)⁵ has a horizontal tangent at point P with a x coordinate 3/8. Show that the y coordinate of P is 3³ x 5⁵ /8⁸.
Sorry guys I have absolutely no idea. Was thinking along the lines of the product rule, but not sure. Help with working out appreciated.