Can someone pls pls pls help me with this questions asap! Thanks so much

Edgar

Let L = C(s,t) where s and t are indeterminates (or more formally L is the field of fractions of C[s,t]). Let Φ: L → L be given by Φ(s) = t, Φ(t) = s, and let G be the group generated by Φ. Find the fixed field of G, justifying your answer.