how to find the midpoint of a line ona graph

Technically, a line doesn't have a midpoint, since its length is infinite....

But, if you're talking about a segment, we just add the x coordinates of the endpoints and divide this by 2, and then do the same with the y coordinates of the endpoints, thusly:

midpoint of a segment = ( [ x₁ + x₂ ] /2, [y₁ + y₂] / 2 )   ......where (x₁, y₁), (x₂, y₂) are the segment endpoints