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(-1)=1(-1)+6=5; then f(g(-1))=-3(5)- 5=-20. This is why the final value of (f∘g) is obtained only after the second substitution, not after finding g alone.