Answer
$=-\displaystyle \frac{3}{x(x+1)}, \quad x\neq-1,0$
Work Step by Step
The denominators are different.
Find a common denominator.
LCD=$x(x+1)$
$\displaystyle \frac{3}{x+1}-\frac{3}{x}=\frac{3x}{(x+1)\cdot x}-\frac{3(x+1)}{x(x+1)}$
$=\displaystyle \frac{3x-3x-3}{x(x+1)}$
$=-\displaystyle \frac{3}{x(x+1)}$
... exclude values that yield 0 in the denominator:
$x\neq-1,0$
... and, cancel common factors (here, there are none)
$=-\displaystyle \frac{3}{x(x+1)}, \quad x\neq-1,0$
