$a)$

$2^{-6}=\frac{1}{2^6}=\frac{1}{4^3}=\frac{1}{64}$

$5^{-5}=\frac{1}{5^5}=\frac{1}{25\cdot 25\cdot 5}=\frac{1}{625\cdot 5}=\frac{1}{3125}$

${10}^{-9}=\frac{1}{{10}^9}=\frac{1}{1000000000}$