$($a$)(\frac{x^7}{x^4{x}^{-1}}{)}^6=x^{?}$

$($b$)\frac{1}{(x-y{)}^6}((x-y{)}^3{)}^{-3}=(x-y{)}^{?}$

$($c$)\frac{(x-a{)}^{-2}}{(x-a{)}^{-5}}(x-a{)}^0=(x-a{)}^{?}$