Find the equation of the tangent line to the function f(x) = 3cos 2x – 1 at the point M(п/2,-1)

As Melody has said, the slope when x = pi/2 = 0.....and since the slope of the tangent line is 0, the line is just a horizontal one and will be given by [ y = the y coordinate of the point (pi/2, -4) ], i.e.,   y = -4

Here's the graph of the function and the tangent line.....https://www.desmos.com/calculator/0ekn0psfnf

That point is not on the graph so this will be difficult.  

However 

y'=-6sin(2x)

When x= pi/2

Y'=-6 × 0 = 0

Incidentally y will equal -3-1 = -4

Thanks for your attempt, I also got the result -4... but I need an equation for it