Find the equation of the tangent line to the curve y3xcosx a

Find the equation of the tangent line to the curve y=3xcosx at the point (pi, -3pi).
The equation of this tangent line can be written in the form y=mx+b where

m=?

and

b=?

I have m as -3 but part b was not -6pi , so I need help to find out what b is.