Calculate using power formulas.

a) [[$ \dfrac{6^{-15}}{6^{-14}}$]]

b) [[$ (6^{-1})^{-2}$]]

c) [[$ \dfrac{4^{-6}}{4^{-8} \cdot 4^2}$]]

d) [[$ \dfrac{2^2}{2^{-5} \cdot 2^3}$]]