$$\boxed{If\;\;y=a^x\;\;then\;\;\frac{dy}{dx}=(lna)a^x}$$
Mmm, I didn't know this short cut Chris, I will have to try and remember it! Wish me luck. :)
This is how I would have done it,
$$\\y=6.687*0.9316^x\\\\
\frac{y}{6.687}=0.9316^x\\\\
ln(\frac{y}{6.687})=ln(0.9316^x)\\\\
ln(y)-ln(6.6870)=xln(0.9316)\\\\
\frac{1}{y}\;\frac{dy}{dx}-0=ln(0.9316)\\\\
\frac{dy}{dx}=yln(0.9316)\\\\
$sub in y$\\\\
\frac{dy}{dx}=6.687*0.9316^x*ln(0.9316)\\\\
\frac{dy}{dx}=6.687*ln(0.9316)*0.9316^x\\\\$$
.
