I'm new here so maybe I'll need some help with formatting with MathJax, as well.
So question asks for tangent line of $f(x) = x\cos(3x), x= \pi$
$$f(x) = x.\cos(3x)$$ $$f'(x) = -x.\sin(3x)+\cos(3x).3$$
$$f(\pi) = -\pi$$ $$f'(\pi) = -3$$
The answer I found:
$$y = -3x - 2\pi$$
The answer of the book: $$y = -x$$
Considering: $$y = ax + b$$ $$y = f(x)$$ $$a = f'(x)$$ What am I missing?