Answer
$$\left| {{P_1}{P_2}} \right| = \sqrt {{{\left( {{x_2} - {x_1}} \right)}^2} + {{\left( {{y_2} - {y_1}} \right)}^2}} $$
Work Step by Step
$$\eqalign{
& {\text{Suppose that we know the points }}{P_1}\left( {{x_1},{y_1}} \right){\text{ and }}{P_2}\left( {{x_2},{y_2}} \right) \cr
& {\text{We can calculate the distance between them using the }} \cr
& {\text{distance formula}}: \cr
& \left| {{P_1}{P_2}} \right| = \sqrt {{{\left( {{x_2} - {x_1}} \right)}^2} + {{\left( {{y_2} - {y_1}} \right)}^2}} \cr} $$
