A rectangle has two adjacent vertices at (−1, 2) and (5, 2), and its area is 30 sq units. Which could be its other two vertices?

Side along $y=2$ has length $5-(-1)=6$, so the other side is 30 ÷ 6 = 5. The rectangle goes 5 up $(y=7)$ or 5 down $(y=-3)$; only (5, −3), (−1, −3) is offered as a matching pair. (5, 5) gives height 3, area 18; mixing 7 and −3 does not make a rectangle.