Doubt M.G.Calkin exercise

I have a doubt with the first exercise from the book M.G.Calkin (Lagrangian and Hamiltonian mechanics). Maybe someone who knows can help me. In Exercise 1.01, in the solution they set "sin (PI * x/2a) = Root (E / (E + Vo)) sin (f)". I do not understand how it has come to that equality.

Thank you