The below picture is actually a top-down view of a 3d cube. The corners on the cube are labelled points A, B, C, and D.

My question is: If I know that the vector AC = (-11, 26, 8) and BD = (-26, 11, 0), how can I find the 3d coordinates for point D? In other words: B = (x, y, 0), find x and y.

I know it involves halfing both vectors to obtain the midpoint of CA and BD, just not sure how to use that information to find x and y.

EDIT: If helpful, point C = (2, 4, 17).