Question 980036
Hi there,
If you give the line points.
A (you are trying to find)
B (Mid pt) (6,2)
C (End pt) (9,-2)

AB/BC = b - a/c - b = 1/1
Cross multiply
b - a = c - b
-a = c - b - b
-a = c -2b
Multiply both sides by -1
a = 2b - c
a = 2(6,2) - (9,-2)
a = (12,4) - (9,-2)
a = (3, 6)
Therefore point A = (3,6) 
Hope this helps:-)