Hi, everyone. First post but I have a feeling I'll be coming back often. (Hi)

Problem: For what values of in the interval does the graph of have a horizontal tangent?

Answer:

f(x) has horizontal tangents at

I don't have a problem getting the x-values for the horizontal tangents but I can't get the right derivative. Here's my work:

,

,

Substitute back into the equation.

Where is my error?

Thank you for your help,

Tim