Explanation:
the area of a rectangle is
length × width.
in our case that is
(7/3 × b + 21) × (3/7 × b) =
= 7/3 × 3/7 × b × b + 21 × 3/7 × b =
= 1 × b² + 3×3 × b = b² + 9b = b(b + 9) ft²
so, the area is
b² + 9b = b(b + 9) ft²
remember, an area is always a square "something".
a volume a cubic "something".
so, when the lengths are given in feet, the areas are square feet or ft².