It seems that your formula is misunderstood.

In the title, x and y should be coordinates, and a, b, c and d should be corresponding points, indicating subscripts.

Xa represents the abscissa of point a. ....

The third problem in the figure below is not handled by this formula, but should be handled in rectangular coordinate system:

Use the equal abscissa difference and equal ordinate difference of two groups of adjacent points of parallelogram to find point P.