Hi !
To calculate a distance between two points A and B of a coordinate plan, we use this specific fomula : [tex]\boxed{\boxed{AB=\sqrt{\left(x_B-x_A\right)^2+\left(y_B-y_A\right)^2}}}[/tex]
So lets say we have two points : A(0, 3) and B (5, -9)
[tex]AB=\sqrt{\left(5-0\right)^2+\left(-9-3\right)^2}\\\\AB=\sqrt{5^2+(-12)^2}\\\\AB=\sqrt{25+144}\\\\AB=\sqrt{169}\\\\\boxed{AB=13}[/tex]