what is the cost to install new carpet in an 15' by 21' room if the price of the carpet including installation is $14.50 a square yard?

Answer 1

Just figure the area of the room:
area = 15'*21' = 315 ft^2
one yd^2 = (3ft)^2 = 9 ft^2
so, area = 315/9 yd^2 = 35 yd^2

now you can take the last step:

cost = 35 yd^2 * $14.50/yd^2 = $507.50