The answer should be 84.
Since you are doing combo's you need to use it's formula. n! / r!(n-r)! n=9 r=3
Plug in your numbers and solve. The ! symbols just means you are doing that number times each number below it. So when you plug in 9 in for n in the numerator of your formula you get 9!, which means 9x8x7x6x5x4x3x2x1. I'll leave the rest to you.