Answer
$\displaystyle \frac{x-2}{x-7}$
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.
$\displaystyle \frac{3-x}{x-7}-\frac{2x-5}{7-x}\cdot \displaystyle \frac{-1}{-1} = \displaystyle \frac{3-x}{x-7}-\frac{-(2x-5)}{-(7-x)}$
$= \displaystyle \frac{3-x}{x-7}-\frac{-(2x-5)}{x-7} \qquad ... -\displaystyle \frac{-A}{B}=+\frac{A}{B}$
$= \displaystyle \frac{3-x}{x-7}+\frac{2x-5}{x-7}$
... To add/subtract rational expressions with the same denominator,
add/subtract numerators and place the sum/difference over the common denominator.
$= \displaystyle \frac{3-x+2x-5}{x-7}$
= $\displaystyle \frac{x-2}{x-7}$