width = 10 feet → C
let l represent the length then width = l - 8
perimeter = 2(length + width) = 2(l + l - 8)
given perimeter = 56 then
2(2l - 8)= 56
divide both sides by 2
2l - 8 = [tex]\frac{56}{2}[/tex] = 28
add 8 to both sides
2l = 28 + 8 = 36
divide both sides by 2
l = [tex]\frac{36}{2}[/tex] = 18
thus length = 18 feet and width = 18 - 8 = 10 feet
