Take the formula of a rectangle and use substition to fill in the information you already know.
A= LxW
57f = L x 3.8f
Now use the balance method to figure out what the length is. You're really just doing algebra.
57f= L x 3.8f
57f ÷ 3.8 f = L x 3.8f ÷ 3.8f
15f= L
ANSWER: The length of the sidewalk in 15 feet.
CHECK:
A= L x W
A= 15f x 3.8f
A= 57 feet