Energy levels A, B, C of a certain atom corresponding to increasing values of energy i.e. EA

Let the energy in A, B and C state be EA, EB and EC, then from the figureEC−EB+EB−EA=EC−EA  ⇒hcλ1+hcλ2=hcλ3 ⇒λ3=λ1λ2λ1+λ2