Answer:
25 yards
Explanation:
A parallelogram is a quadrilateral (a four sided object) with parallel sides 
Some of the characteristics of a parallelogram includes : 
- Opposite sides and angles are congruent . 
- Opposite angels are congruent (D = B). 
- Consecutive angles are supplementary (A + D = 180°). 
- The diagonals bisect each other
Area of a parallelogram = base x height 
Area = 525
base = 21 yards
height = ?
525 = 21 x height 
divide both sides of the equation by 21 to find the value of the height 
525 / 21 = height 
height = 25 yards