We will investgate the cost of a driveway on square foot basis.
A driveway is modeled as a rectangle with elementary side lengths defined as follows:

The rate for contructing a driveway per square foot is gvien as follows:

To determine the total cost of the driveway with the defined side lengths we need to consider the area ( A ) covered by the entire driveway. We will model the driveway in shape of a rectangle with the following area:

Plug in the respective side-lengths and determine the area ( A ) as follows:

Next we will determine the total cost of constructing a driveway with a covered area ( A ) as follows:

Plug in the respective quantities and evaluate the cost or the amount Kelly pays:

Hence, Kelly pays $2,100!