Answer:
![315\ ft^2](https://img.qammunity.org/2020/formulas/mathematics/middle-school/soqvpbvmcfb4s309xv47ve5xta19jwsev2.png)
Explanation:
In order to calculate the area of a rectangle, you can use the following formula:
![A=lw](https://img.qammunity.org/2020/formulas/mathematics/middle-school/6ha4hehcoeuii45f92n9qa4p9h4g0r5n0e.png)
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:
![l=21\ ft\\w=15\ ft](https://img.qammunity.org/2020/formulas/mathematics/middle-school/dd2e62eyth7ocst9xugijmw0ebblfpnqo0.png)
Therefore, you can substitute these values into the formula, getting that the area of this rectangle is:
![A=(21\ ft)(15\ ft)\\\\A=315\ ft^2](https://img.qammunity.org/2020/formulas/mathematics/middle-school/wtau3pcvujndw0lpas65sszwt8du41x6tu.png)