Question

wallpaper cost $2 per square yard how much would it cost to cover a rectangular wall that measures 15 feet by 21 feet

Answer 1

The area of the wall is (15-ft x 21-ft) = 315 square feet

That's the easy part.  Now you have to figure out how many
square yards that is.

9 square feet = 1 square yard.

315 square feet = 315/9 = 35 square yards.

The cost of the wallpaper is (35) x ($2) = $70 .