sqrt{(x_{2}-x_{1})^{2}+(y_{2}-y_{1})^{2}

For example, you have the coordinates of (5, 4) and (-3, 8)

sqrt{(4-8)^{2}+(5-(-3))^{2}

sqrt{-4^{2}+8^{2}

sqrt{16+64

sqrt{80

4sqrt{5

Most always, people answer within a day or two.