You did part (a) correctly!
Part (b):
If [tex]h = g \circ f,[/tex] then [tex]h^{-1} = f^{-1} \circ g^{-1}.[/tex]
Look at the boundaries of G. They will give you the boundaries of its inverse image. This isn't ALWAYS the case, but our function here is pretty simple so it will work out well.