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