Answer:
 L  = 2r  + 3
Step-by-step explanation:
Given parameters:
Area of rectangle  = 14r + 21
Width of the rectangle  = 7ft
Unknown:
Length of the rectangle  = ?
Solution:
If the length of the rectangle is designated as L;
 Area of a rectangle = Length x width
Now insert the parameters:
      14r + 21  = L x 7
      L  = [tex]\frac{14r + 21}{7}[/tex] Â
      L  = 2r  + 3