The y is 2 (same as co-ord C and drawing the rhombus)
The X is -2+5=3 (length of side is 5. again, using the coord of C and drawing the rhombus)
hence coordinate is (2,3)
Re: A rhombus with vertices A (1,6) B (6,6) C (-2, 2) and D (x, y)
Find the midpoint of B(6,6) and C(-2,2) (two opposing vertices)
This midpoint should also be the midpoint of A(1,6) and D(x,y) and you can find it from there
Just looking at it, I think D should be (3,2)