**HINT**
We have that
$$\left(\frac{x+2}{x-6}\right)^{\frac x 4}=\left(\frac{x-6+8}{x-6}\right)^{\frac x 4}=\left(1+\frac{8}{x-6}\right)^{\frac x 4}$$
then we can manipulate further in order to use the standard limit.
Refer to the related
* Calculating a limit with trignonometeric and quadratic function