You need to simplify using the laws of indices.
Recall that,
[tex] {a}^{ - m} = \frac{1}{ {a}^{m} } [/tex]
We apply this property to obtain,
[tex] {9}^{ - 2} = \frac{1}{ {9}^{2} } [/tex]
Recall again that,
[tex] {a}^{2} = a \times a[/tex]
Therefore our expression become,
[tex] {9}^{ - 2} = \frac{1}{ 9 \times 9 } [/tex]
This gives us,
[tex] {9}^{ - 2} = \frac{1}{ 81 } [/tex]
You should also take note of this property too.
[tex] {a}^{ m} = \frac{1}{ {a}^{-m} } [/tex]
[tex]<i>This means that the sign of the index changes whenever you reciprocate a number that is raised to a given index</i>[/tex]
