Answer
$\bf{\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}}$
Work Step by Step
The distance $d$ between two points $P_1(x_1,y_1)$ and $P_2(x_2,y_2)$ on the xy -plane can be expressed by the following formula:
$d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}$.
Therefore, the missing term in the given statement is $\bf{\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}}$.