#### Answer

$\dfrac{y_2-y_1}{x_2-x_1}$

#### Work Step by Step

RECALL:
The slope of a line can be found using the formula $m=\dfrac{y_2-y_1}{x_2-x_1}$ where $(x_1, y_1)$ and $(x_2, y_2)$ are points on the line.
Thus, the missing expression in the given statement is:
$\dfrac{y_2-y_1}{x_2-x_1}$