Derivative of cosx-3x

1. Differentiate $- 3 x + \cos{\left(x \right)}$ term by term:

   1. The derivative of cosine is negative sine:

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

   2. 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: $-3$

   The result is: $- \sin{\left(x \right)} - 3$

The answer is:

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