Answer: [tex]315\ ft^2[/tex]
Step-by-step explanation:
In order to calculate the area of a rectangle, you can use the following formula:
[tex]A=lw[/tex]
Where "l" is the lenght of the rectangle and "w" is the width.
In this case you know that this rectangle has a length of 21 feet and a width of 15 feet. Then:
[tex]l=21\ ft\\w=15\ ft[/tex]
Therefore, you can substitute these values into the formula, getting that the area of this rectangle is:
[tex]A=(21\ ft)(15\ ft)\\\\A=315\ ft^2[/tex]