How do you use the graph of f(x)=6^-x to describe the transformation of g(x)=6^x?

Take $g \left(x\right) = f \left(- x\right)$
When we perform the transformation $f \left(- x\right)$, we change the sign of every $x$ in our function.
So if we want $g \left(x\right) = {6}^{x}$ from $f \left(x\right) = {6}^{- x}$, we need $g \left(x\right) = {6}^{- - x} \equiv f \left(- x\right) .$