How do I find out the coordinates of the fourth point of a parallelogram knowing the first three?

Say the parallelogram is called ABCD, you know the coordinates of A, B and C and want to find out DFirst of all, you'll want to calculate a vector from one of the sides of the parallelogram, say AB. Because this is a parallelogram, opposite vectors are equal, meaning AB = CD. This means that you know the x- and y- components of CD, but you also know that x_CD= x_D- x_C and y_CD= y_D- y_C. By rearranging, you can find x_D and y_D, the coordinates of D.