sum_{k=0}^{b} binom(j+k, j)*binom(a+b-j-k, b-k) * y^(b-k) * z^k

Uh oh! Wolfram|Alpha doesn't run without JavaScript.

Please enable JavaScript. If you don't know how, you can find instructionshere.Once you've done that, refresh this page to start using Wolfram|Alpha.