Slope = (-12 - 7) / ( 3 - - 1) = -19 /4
So.....the slope between all points is the same......
( -12 - y) / ( 3 - - 6) = -19/4
(-12 - y ) / ( 9) = -19/4
- ( 12 + y) / 9 = -19/4
(12 + y) / 9 = 19/4
12 + y = 9 * 19 /4
y = 9 (19/4) - 12
y = 171 / 4 - 48 / 4
y = 123 / 4
