Solve for solutions over the interval [0,360)

Okay, so you got as far as \(\displaystyle cos(3x)=-1\) and you're confused as to where to go next. What have you learned about the inverse cosine function? What happens if you take the inverse cosine of both sides? \(\displaystyle cos^{-1}(cos(3x))=cos^{-1}(-1)\). Where does that lead?
 
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