Find the derivative of $LaTeX: \displaystyle f(x)=9 \sin{\left(x \right)} + 9 \cos{\left(x \right)}$ .

Using the sum rule to take the derivative of each term separately gives $LaTeX: \displaystyle f'(x)=- 9 \sin{\left(x \right)} + 9 \cos{\left(x \right)}$ .