The diagram shows a possible rectangle. To find the coordinates of each corner of the rectangle we use the fact that to get from A to B we move 20 units to the right. This means we add 20 to the x coordinate and zero to the y coordinate giving us (23,9) for B. To get from B to C we move down 5 units which means we take 5 off the y coordinate and leave the x coordinate alone. So we go from (23,9) to (23,9-5)=(23,4) for C.
Your picture may be different from mine, but if you understand the procedure you should be able to work out the coordinates for yourself by adding or subtracting the lengths of the sides of the rectangle.