text: If order matters, if RHO is not the same as ORH, then you would do a permutation.  There are 7 letters in rhombus, so 7 taken 3 at a time is 7P3 = 7!/4! or [7*6*5..*2*1]/[4*3*2*1] = 7*6*5 = 210.\n\nBasically, you have just found not only all the combinations of letters that can be pulled out, but also all the different ways they can be arranged.  If the order doesn't matter, that is RHO is the same as ORH, then you need a combination.\n\n7C3 = 7!/(3!)(4!).  You already found 7!/4!, which was 210.  Now dividing by 3! will undo all the different arrangements of the letters and give you 35 combinations.