Kinetic energy of electron in first excited state = 3.4 eV
de-Broglie wavelength of electron having kinetic energy K, \(\lambda = \frac{h}{\sqrt{2 Km}}\)
de-Broglie wavelength of electron in first excited state \(\lambda = \frac{6.63 \times 10^{-34}}{\sqrt{2 \times 3.4 \times 1.6 \times 10^{-19}\times9.1 \times 10^{-31}}}\)
\(\lambda = \frac{6.63 \times 10^{-34}}{\sqrt{99.008\times 10^{-25}}}\)
\(= 6.66 \times 10 ^{-10} m = 6.66 \mathring{A}\)
