The midpoint of a line joining two points is the average of their coordinates: ((1+19)/2, (2+4)/2)=(10,3).
Gradient of the line: (7-(-3))/(0-4))=10/(-4)=-5/2. The slope of the perpendicular is the negative reciprocal of this=2/5.
Standard form of the required line is y=mx+b. Plug in (10,3) and m=2/5:
y-3=⅖(x-10)=2x/5-4, so y=2x/5-4+3=2x/5-1 is the equation of the perpendicular.
When x=20, y=7.