refer to the first and second points; it doesn't matter which points you call
first or second.
The distance formula can be obtained by creating a triangle and
using the Pythagorean Theorem to find the length of the hypotenuse. The hypotenuse
of the triangle will be the distance between the two points.
x2 and y2 are the x,y coordinates for one point
x1and y1 are the x,y coordinates for the second point
d is the distance between the two points