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A power with the exponent $21 $ can be rewritten as the square root of the base. In other words, the square root of some number is equal to $3.$ We know that $3⋅3=9,$ and so we are looking for $9 .$ Now, we need to write it as a power.
$9 =9_{1/2}.$
Thus, the number $3$ can be written as the power $9_{21}.$
### Alternative Solution

Alternative Solution

We are looking for a number which we have to take the square root of to get the number $3.$ This can be expressed with the equation $a =3,$ where $a$ is the number we are looking for. By squaring both sides, we can see which number we need to raise to the power of $21 $ to end up with $3.4$

We have found that the number $3$ can be written as $9_{21}.$