The next number would be 243.
3 x 7 = 21
9 x 8 = 72
_ x 9 = __
In this pattern, the second number seems to be consecutive integers. The first number seems to be multiples of 3. 3^1 is 3, and 3^2 is 9, so the next number would have to be 3^3 times 9. This would be 27 x 9 = 243.