Given :
 .
 .
B is the mid-point of AC .
To Find :
The missing endpoint if B is the midpoint of AC.
Solution :
Let , coordinate of point A is ( h, k) .
So , mid-point B is given by :

Comparing above point with 
 , we get :
 , we get :
 
 

Therefore , the value of point A is 
 .
 .
Hence , this is the required solution .