You'll need to use a triple integral (at least, that's the easiest way imo). I would do it in the order of dydxdz, but that's up to you. Your limits on y would be 0 to sqrt(1296-x^2), your limits on x would be found be using 0 = sqrt(1296 - x^2) and solving for x, and your limits on z would be -36 to 36. I believe this would be the correct answer, someone correct me if I'm wrong.