Looking over a practice exam for my midterm on Friday...I came across the following question:

Evaluate C(21,2) using Pascal's Identity.

I have written down in my notes that Pascals identity means that C(n,k) + C(n,k-1) = C(n+1,k) for 1 <= k <= n+1 & for any natural number.
Example: C(4,2) + C(4,3) = C(5,3)

How might I go about using this to evaluate something? I don't understand what the question is asking...