We know that wavelength is inversely proportional to momentum, let λB is de-Broglie Wavelength corresponds to momentum PB, similarly let λG is Wavelength at ground state corresponds to momentumPG,
λGλB=PBPG=mvBmvG, and velocity, V∝nz, where Z is the atomic number, and n is a number of orbits, here, nG=1 (ground state), nB=3 (second excited state). So, λGλB=nGnB=13⇒λB=3λG