Select 'Graph' tab, type in the function and click on 'Solve'.
You'll get the following graph:

Click here to see the graph.

You can see the y-intercepts on the graph. Ad you can see, they are the solutions of the equation:

That is, when y = 0

Click here to see the step by step solution for the equation.

The value of the derivative function for any value 'x' is the slope of the original function at 'x':

y' = 9x^2 - 8x - 15