Factoring

I'm having a lot of trouble with this simple concept, can someone walk me through factoring something like this?

3x^2 -4x-15

I have a program that is supposed to explain this sort of thing to me, but its "help" is entirely useless:

http://imgur.com/jXprS

I need to learn this stuff, not knowing how to do it has been holding me back in class. Thanks very much in advance.