The given equation is,
[tex]y=3\cos (\frac{x}{4})-6[/tex]
The graph can be drawn as,
The value of y varies from -3 to -9.
The equation of midline can be determined as,
[tex]\begin{gathered} y=\frac{-3+(-9)}{2} \\ =-6 \end{gathered}[/tex]
Thus, the required equation is y=-6.
