ANSWER
[tex]\frac{1}{3^2a^3}[/tex]EXPLANATION
We are given the expression:
[tex]3^{-2}a^{-3}[/tex]To write this as a fraction, we must note that from the laws of indices:
[tex]x^{-y}=\frac{1}{x^y}_{}[/tex]This therefore implies that:
[tex]3^{-2}a^{-3}\text{ = }\frac{1}{3^2a^3}[/tex]That is the expression written as a fraction.