Formula
The midpoint $M$ is found by averaging the x-coordinates and the y-coordinates of the two points.
M = ( (x₁ + x₂) / 2 , (y₁ + y₂) / 2 )
The Midpoint Formula
Distance
While finding the center, you can also find the length of the segment.
d = √(x₂ - x₁)² + (y₂ - y₁)²