How do you find the points where the graph of the function #y=(x+3)(x-3)# has horizontal tangents?
Observe that horizontal tangents occur at a min/max.
You don't need Calculus to answer this question:
The function is a quadratic with roots given by:
As the coefficient of
So there is a horizontal tangent at
If you should want to use Calculus then:
To establish the nature of the critical point we look at the second derivative: