(s 6 d/rt 7 ) ​-8

$\left(\dfrac{s^6d}{rt^7}\right)^{-8}\\ = \left(\dfrac{rt^7}{s^6d}\right)^8\\ = \left (\dfrac{r^8t^{56}}{s^{48}d^8} \right )$

That could not be further simplified.