Which number completes the analogy? 63 : 9 :: 84 : ?

The two digits are added together: 6 + 3 = 9, so 8 + 4 = 12. The trap 32 multiplies the digits instead, but $6\times 3=18$ and not 9, so that rule dies on the first pair. 4 takes the difference of the digits, which would give 3 rather than 9 the first time, and 21 is simply 12 with its digits turned round.