Answer
$g'(x)=\frac{-3}{\sqrt{4-x^2}}$
Work Step by Step
$g(x)=3\arccos(\frac{x}{2})$
$let$
$u=\frac{x}{2} , u=\frac{1}{2}$
$\frac{d}{dx}(\arccos u)= \frac{-u'}{\sqrt{1-u^2}}$
$g'(u)=3\frac{-u'}{\sqrt{1-u^2}}$
$g'(x)=\frac{-3}{2\sqrt{1-\frac{x^2}{4}}}$
$g'(x)=\frac{-3}{\sqrt{4-x^2}}$