Given two points [tex] A = (A_x, A_y) [/tex] and [tex] B = (B_x, B_y) [/tex], you can compute their distance using the Pythagorean theorem: you have
[tex] AB = \sqrt{(A_x-B_x)^2 + (A_y - B_y)^2} [/tex]
In your case, you have
[tex] AB = \sqrt{(-1-1)^2 + (-1-3)^2} = \sqrt{(-2)^2+(-4)^2} = \sqrt{4+16} = \sqrt{20} = 2\sqrt{5} [/tex]
