Have you tried to this yourself? What problem did you have? You are told exactly what to do. What are ||f|| and ||g||? What are ||f+ g|| and ||f- g||? What is ?
Surely you see that f+ g= 1 for all x in [0, 2] and that f- g= 2x-1 for x in [0, 1] and 3- 2x for x in [1, 2].