The coordinates of the fourth vertex will be (8, 5). The coordinates (6, 3) and (6, 5) are on top of each other with (6, 5) being two units above (6, 3). The coordinate (8, 3) will be two units to the right of (6, 3). To make a rectangle, there are two pairs of the same length, therefore the fourth coordinate must be (8, 5).