Chapter 9 - Polar Coordinates; Vectors - 9.6 Vectors in Space - 9.6 Assess Your Understanding - Page 624: 1

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

The distance $d$ from $P_1(x_1,y_1)$ to $P_2(x_2,y_2)$ is given by the formula $d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}$. Thus, the missing expression in the given statement is $\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}$.