Answer
See below.
Work Step by Step
The numerator will be: $x-(x+3)=-3$, thus the fraction will be $\frac{-3}{3x(x+3)}=\frac{-1}{x(x+3)}$
This is because $\frac{x(x+3)}{x(x+3)}\frac{\frac{1}{x+3}-\frac{1}{x}}{3}=1\frac{\frac{x}{x(x+3)}-\frac{x+3}{x(x+3)}}{3}=\frac{x-(x+3)}{3x(x+3)}=\frac{-3}{3x(x+3)}=\frac{-1}{x(x+3)}$
