The rule you use is x^(-k) = 1/(x^k)
In this case x = 3 and k = 2, so,
3^(-2) = 1/(3^2) = 1/9
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One way to think of it, is to consider the equation below
3^2*3^x = 1
We can solve for x like this
3^2*3^x = 1
3^(2+x) = 1
3^(2+x) = 3^0
2+x = 0 .... the bases are equal, so the exponents must be equal
x = -2
meaning that
3^2*3^x = 1
updates to
3^2*3^(-2) = 1
Now isolate the 3^(-2)
3^2*3^(-2) = 1
3^(-2) = 1/(3^2)
3^(-2) = 1/9