To solve this problem you must apply the proccedure shown below:
1- You have that the area of the living room is:
A=lw (Area of the rectangle); where l is the lengt and w is the width.
A=(21 feet)(15 feet)
A=315 feet²
2- The cost is:
Cost=315 feet²x$9
Cost=$2835
The answer is: He will need 315 feet² and it will cost $2835