Answer
$\displaystyle \frac{12}{x+3}$
Work Step by Step
When one denominator is the opposite, or additive inverse of the other,
first multiply either rational expression by $\displaystyle \frac{-1}{-1}$
to obtain a common denominator.
$\displaystyle \frac{10}{x+3}-\frac{2}{-x-3}\cdot \displaystyle \frac{-1}{-1} =\displaystyle \frac{10}{x+3}-\frac{-2}{-(-x-3)}$
$=\displaystyle \frac{10}{x+3}-\frac{-2}{x+3} \qquad ... -\displaystyle \frac{-A}{B}=+\frac{A}{B}$
$=\displaystyle \frac{10}{x+3}+\frac{2}{x+3}$
... To add/subtract rational expressions with the same denominator,
add/subtract numerators and place the sum/difference over the common denominator.
=$\displaystyle \frac{12}{x+3}$
