Need help with a an unfamiliar mathematical operation

Question

What does (9 over 3) mean? I can't type the exact operation; please see attached. Or can you direct me to a site where this operation is explained?

Answer 1

This is a notation that stands for "9 choose 3"

You probably know it as $${9 \choose 3 }= \ _9C_3$$

Answer 2

$\dbinom 9 3$, pronounced "nine choose three", is the number of subsets of size $3$ of a set of size $9$.

This can be shown to be $\dfrac{9\cdot8\cdot7}{3\cdot2\cdot1} = 3\cdot4\cdot7=84$.