Help

The midline is the y value that runs straight through the middle of the wave.If you can picture the standard sine function graph.  f(x) = Sin(x) reaches a maximum y = 1 and minimum y = -1 so the midline is y =0. This equation is shifted up 6 so the midline is shifted up 6y = 6 is equation for midline

Equation of midline  ...  y =  [ max value of function + min value of function ]  / 2

Note that the max value of the cosine = 1    and the min value  = -1

So....max value of function =  3(1) - 2.5  =   .5

And the min value of the function is  3(-1) - 2.5 = -5.5

So the equation of the midline is

y =   [ .5  +    -5.5] / 2   =  [ -5  ] / 2   =   - 2.5

See the graph here :  https://www.desmos.com/calculator/lesls9edag