Algorithm of 10242 - Fourth Point !!

Problem Description Link
Algorithm:

To solve this problem firstly you need to find the common   (x, y) coordinates,  then you find other two point .
After that you need find a,b
ie. common point (x2,y2) and other two point (x1,y1) and(x3,y3).
       a=(x1+x3)/2;
        b=(y1+y3)/2;
        x4=2*a-x2;
        y4=2*b-y2;