how do you figure this out?

All im asking is how you figure out the odds of you hole cards hitting certain cards on the flop. For example, how do i figure out the odds of a 33 hitting a 3 on the flop, stuff like that(and i already know its about 7-1). I jsut want a formula that is used to figure this stuff out, or maybe a link to a chart somewhere on the internet that has it all mapped out for me already (i would prefer a formuola though).