how to write (3.5,1.5) as a midpoint ?
well, it is the midpoint of the line connecting
(3,1) and (4,2)
or
(3.5,-20) and (3.5,-17)
or
(-4,1.5) and (11,1.5)
among many others.
(3,1) and (4,2)
or
(3.5,-20) and (3.5,-17)
or
(-4,1.5) and (11,1.5)
among many others.