The sub-duplicate ratio of $x$ : 81 is 2 : 3. The value of $x$ is:

The sub-duplicate ratio of a : b is √a : √b, so √$x$ : √81 = 2 : 3 → √$x/9=2/3$ → √$x=6$ → $x=36$. (Solving $x$ : 81 = 2 : 3 straight off gives the trap 54 and ignores the square roots.)