Only expressions can be factored. If you want to factor 'x^3-2x^2-5x+6' you need to type down the expression 'x^3-2x^2-5x+6' instead of the equation 'y = x^3-2x^2-5x+6'.

This way you will get:

x^3-2x^2-5x+6 = (x - 3)(x - 1)(x + 2) 

So the zeros are: 3, 1 and -2.

If you want to graph, then you can type 'y = x^3-2x^2-5x+6' under the 'Graph' tab and read the zeros from the graps:

Click here to see the graph.