Let (x) = ax + b and g(x) = cx + d. What condition must be satisfied by the constants a, b, c, d in

Let ƒ(x) = ax + b and g(x) = cx + d. What condition must be satisfied by the constants a, b, c, d in order that (ƒ ∘ g)(x) = (g ∘ ƒ)(x) for every value of x?