it is not the case that g being 1-1 means g(x1) = x1.
for example, g(x) = x+1 is 1-1, but g(x) is never x.
furthermore, you are not being asked to show that (b) is true. you are being asked to give a formula for (f o g)^-1.
what is the definition of 1-1 in your text?