So:
[tex]a^{-6}=(\frac{1}{a})^6[/tex]Now:
[tex]\frac{a^{-6}}{a}=(\frac{1}{a})^6\cdot\frac{1}{a}=(\frac{1}{a})^{6+1}=(\frac{1}{a})^7=\frac{1}{a^7}[/tex][tex](a^{-3})^2=a^{-3\cdot2}=a^{-6}[/tex][tex]\frac{\mleft(a^{\mleft\{-3\mright\}}\mright)^2}{a^{-3}}=\frac{a^{-6}}{a^{-3}}=\frac{1}{a^6}\cdot\frac{1}{a^{-3}}=\frac{1}{a^{6-3}}=\frac{1}{a^3}[/tex]When you invert a fraction, you change the sign of the exponent.