l = 16 feet
w = 3 feet
A = l×w
l = 10+2w
48 = (10 + 2w)w
48 = 10w + 2w² | : 2
24 = 5w + w²
w² + 5w - 24 = 0
w² + 8w - 3w - 24 = 0
w(w + 8) - 3(w + 8) = 0
(w - 3)(w + 8) = 0
w - 3 = 0 => w = 3 feet
w + 8 = 0 => w = - 8 ∉ N
l = 10 + 2w
l = 10 + 6
l = 16 feet
A = l×w = 16ft×3ft = 48ft²