For a composition, work from the inside to the outside. That means evaluate g(x) first, then use that result as the input of f. Here, Composition means f(g(x)). First g(6)=3(6)+0=18; then f(g(6))=-3(18)+8=-46. This is why the final value of (f∘g) is obtained only after the second substitution, not after finding g alone.