Answer:
D. 21, 14
Explanation:
As the question said, a parallelogram is formed when both pairs of opposite sides are congruent. That means that 21 and 3y are equal, as are y + 7 and 2y.
21 = 3y
/3 /3
7 = y
y + 7 = 2y
-2y -2y
-y + 7 = 0
- 7 - 7
-y = -7
* -1 * -1
y = 7
In both instances, y = 7. Now let's substitute that.
21 = 3(7)
21 = 21 is true.
7 + 7 = 2(7)
14 = 14 is true.
With that, the lengths of the opposites sides are 21 & 14, which is D.