Given a general cosine function as shown below
[tex]y=a\cos (bx+c)+d[/tex][tex]\begin{gathered} \text{where,} \\ d\Rightarrow\text{midline of the equation} \end{gathered}[/tex]From the equation provided in the question
[tex]\begin{gathered} y=-9\cos (\frac{\pi}{2}x-6)+8 \\ \end{gathered}[/tex][tex]\begin{gathered} \text{comparing the general cosine formula with the cosine formula given, we have} \\ a=-9 \\ b=\frac{\pi}{2} \\ c=-6 \\ d=8 \end{gathered}[/tex]Since the midline is d, then the midline equation will be
[tex]y=8[/tex]Hence, the midline equation is y = 8
