Derivative of cos(3x-pi)

1. Let $u = 3 x - \pi$.

2. The derivative of cosine is negative sine:

   $$\frac{d}{d u} \cos{\left(u \right)} = - \sin{\left(u \right)}$$

3. Then, apply the chain rule. Multiply by $\frac{d}{d x} \left(3 x - \pi\right)$:

   1. Differentiate $3 x - \pi$ term by term:

      1. The derivative of a constant times a function is the constant times the derivative of the function.

         1. Apply the power rule: $x$ goes to $1$

         So, the result is: $3$

      2. The derivative of the constant $- \pi$ is zero.

      The result is: $3$

   The result of the chain rule is:

   $$3 \sin{\left(3 x \right)}$$

The answer is:

$$3 \sin{\left(3 x \right)}$$