If f(x) = ax + b and g(x) = cx + d, then f[g(x)] – g[f(x)] is equivalent to
f(b) – g(b)
f(c) + g(a)
f(a) – g(c)
f(d) – g(b)
f(g(x)–g(f(x)) = a(cx + d) + b – {c(ax + b) + d}
= ad + b – bc – d = (ad + b) – (bc + d) = f(d) – g(b)