9=3 squared

M+n of 9 is m+n times the integer square of 3.

The square is proposed as (2m+2n) times of 3.

Exponential addition with the same radix power and invariant radix.

So (2m+2n) times of 3 is equal to 2m times of 3 times 2n times of 3.

Equal to 4×4= 16.

The m+n power of the last 9 = 16.