midpoint : (x1 + x2) / 2 , (y1 + y2) / 2
(18,y)...x1 = 18 and y1 = y
(20,-15)...x2 = 20 and y2 = -15
(18 + 20) / 2 , (y - 15) / 2 = (19,-5)
19 , ?
now we need to figure what - 15 divided by 2 = -5
(y - 15) / 2 = -5...multiply both sides by 2
y - 15 = -5 * 2
y - 15 = -10
y = -10 + 15
y = 5
check..
midpoint : (18 + 20) / 2 , (5 - 15) / 2
m = 38/2 , -10/2
m = (19,-5)
so y = 5 <==
