The formula of a distance between two points:
[tex]d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}[/tex]
We have:
[tex]A(3,\ -4)\to x_1=3,\ y_1=-4\\B(-1,\ 3)\to x_2=-1,\ y_2=3[/tex]
Substitute:
[tex]d=\sqrt{(-1-3)^2+(3-(-4))^2}=\sqrt{(-4)^2+7^2}\\\\=\sqrt{16+49}=\sqrt{65}\approx8.062[/tex]
