distance between two points on a coordinate plane

Using the distance formula, $d = \sqrt{(x_2-x_1)^2 + (y_2-y_1)^2}$, we get $d = \sqrt{(15-3)^2 + (16-7)^2} = \sqrt{12^2 + 9^2} = \sqrt{144 + 81} = \sqrt{225} = 15$.