in addition to what WurlyQ said above, i'd like to point out something else.
Originally Posted by bonghead365
What Im having trouble with is trying to calculate the chances of 2 specific cards showing. like if i hold 2 3 5 and i need a 4 and then a A or a 6 to complete my straight. I know that gives me 12 outs, but ...
it seems that you must have misunderstood the concept of "outs". in this case, you would have 12 cards that would somehow slightly improve your hand. Those are not outs because even if you hit one of them, you still do not have a good made hand. Cards are outs for you only if you think that hitting them will make your hand the best. For the example, even if you hit a 4 on the turn, you still do not have a made hand and now you would be needing to hit your outs - namely an A or 6 on the river (and even then, depending on which cards are in your hand and which are on the board, your hand may still be beaten by a higher straight or a flush if one shows).
So here is the math for your specific question:
options for the turn:
A) an ace or 6 comes - chance 8/47
- now you only need a 4 on the river, and the chance for that is 4/46 (you have seen one more card, remember). the total chance for this is 32/(47*46), or under 1.5%
B) a 4 comes - chance 4/47
- in this case you would need an ace or 6 on the river - chance 8/46. guess what, the total chance for this is the same as above, under 1.5%
C) any other card comes - chance 35/47
- here it doesn't really matter what the river will bring, you are not in a good shape. there are exceptions, of course, like getting both the turn and river be 3s to the 3 in your hand and you win, but those are outside the scope of the current analysis.
so you will be good in under 3% of the time total, when either A or B above happens, and then you get another lucky card on the river. (the difference between my number and WurlyQ's above is because he somehow forgot you could also make a higher straight, with a 5-9 or 9-10.)
basically - if you only need one card, but have chances to hit it on EITHER the turn or the river, then you could (approximately) add the probabilities. if you need specific things to happen on BOTH the turn and the river, you need to multiply the probabilities of the events.
pokerstove (as suggested above) will do the math for you - but it is only really useful as an analysis tool after the fact. you should be much better if you knew some approximate ways to do math while at the table, and it seems you have the basics of that good luck!