$a\text{)}{\left(\frac{7}{8}\right)}^{-\frac{2}{3}}={\left({\left(\frac{7}{8}\right)}^{-1}\right)}^{\frac{2}{3}}={\left(\frac{8}{7}\right)}^{\frac{2}{3}}=\sqrt[3]{{\left(\frac{8}{7}\right)}^2}$     

${\left(\frac{7}{8}\right)}^{-\frac{2}{3}}={\left({\left(\frac{7}{8}\right)}^{\frac{2}{3}}\right)}^{-1}=\frac{1}{{\sqrt[3]{\left(\frac{7}{8}\right)}}^2}=\frac{1}{\frac{\sqrt[3]{7^2}}{\sqrt[3]{8^2}}}=\sqrt[3]{{\left(\frac{8}{7}\right)}^2}$  

---

$b\text{)}{\left(\frac{6}{5}\right)}^{-\frac{7}{8}}={\left({\left(\frac{6}{5}\right)}^{-1}\right)}^{\frac{7}{8}}={\left(\frac{5}{6}\right)}^{\frac{7}{8}}=\sqrt[8]{{\left(\frac{5}{6}\right)}^7}$  

${\left(\frac{6}{5}\right)}^{-\frac{7}{8}}={\left({\left(\frac{6}{5}\right)}^{\frac{7}{8}}\right)}^{-1}=\frac{1}{{\sqrt[8]{\left(\frac{6}{5}\right)}}^7}=\frac{1}{\frac{\sqrt[8]{6^7}}{\sqrt[8]{5^7}}}=\sqrt[8]{{\left(\frac{5}{6}\right)}^7}$