Answer
$$ x=20 ^{\circ} , \ 100^{\circ},\ 140^{\circ},\ 220^{\circ},\ 260^{\circ},\ 340^{\circ} $$
Work Step by Step
Given $$ \cos 3x= \frac{1 }{2}$$
From the figure, we find the intersection points between $y= \cos 3x$ and $ y=\frac{1}{2} $; we get
$$ x=20 ^{\circ} , \ 100^{\circ},\ 140^{\circ},\ 220^{\circ},\ 260^{\circ},\ 340^{\circ} $$
