To find the distance between both points you have to apply pythagoras theorem.
First draw both points and form a rigth triangle with the distance between them as the hypothenuse:
The length of the base of the triangle "a" is determined by the difference between the x-coordinates of both points:
The heigth of the triangle "b" is determined by the difference between the y-coordinates of both points:
Now using phytagoras theorem you can calculate the length of the hypotenuse as:
The distance between points (3,7) and (-5,-7) is 2√65