Solve for x:
add 12 to both sides:
[tex]\begin{gathered} -6\cos (x)-12+12=+12 \\ -6\cos (x)=12 \end{gathered}[/tex]divide both sides by -6:
[tex]\begin{gathered} \frac{-6\cos (x)}{-6}=\frac{12}{-6} \\ \cos (x)=-2 \end{gathered}[/tex]Take the inverse cosine of both sides:
[tex]\begin{gathered} x=2\pi n1+\cos ^{-1}(-2) \\ or \\ x=2\pi n2-\cos ^{-1}(-2) \\ n1\in\Z \\ n2\in\Z \end{gathered}[/tex]