#### Answer

$\sqrt{(x_1-x_2)^2+(y_1-y_2)^2}$

#### Work Step by Step

RECALL:
The distance $d$ between two points is given by the formula:
$d=\sqrt{(x_1-x_2)^2+(y_1-y_2)^2},$
where $(x_1, y_1)$ and $(x_2, y_2)$ are the coordinates of the two points.
Thus, the missing expression in the given statement is:
$\sqrt{(x_1-x_2)^2+(y_1-y_2)^2}.$