Answer
$x=4$ $arccos(\frac{y}{6})$
Work Step by Step
$y=6 cos\frac{x}{4}$
Need to solve $x$ in order to find this divide the above equation by 3 both sides.
$cos\frac{x}{4}=\frac{y}{6}$
Apply definition of arccosine.
$\frac{x}{4}=arccos(\frac{y}{6})$
$x=4$ $arccos(\frac{y}{6})$
