straight draw with gaps - odds

[college kid]

I'm just trying to find the odds of making an open-ended striaght draw on the flop with one gap holes cards ranging from 3-5 to Q-T. I know that to do the math I need to account for the fact that you need teh middle card and then either one of the outside cards but I still can't seem to get the math right for some reason even though it seems simple enough. Thanks.