Derivative of y=cos(2x+6)

1. Let $u = 2 x + 6$.

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 $\frac{d}{d x} \left(2 x + 6\right)$:

   1. Differentiate $2 x + 6$ 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: $2$

      2. The derivative of the constant $6$ is zero.

      The result is: $2$

   The result of the chain rule is:

   $$- 2 \sin{\left(2 x + 6 \right)}$$

4. Now simplify:

   $$- 2 \sin{\left(2 x + 6 \right)}$$

The answer is:

$$- 2 \sin{\left(2 x + 6 \right)}$$