If the radius of first Bohr orbit is a, then de-Broglie wavelength of electron in 3rd orbit is nearly.

According to Bohr theorymvr=nh2π∴       hmv=2πrnAccording  to de−Broglieλ=hmv∴         λ=2πrnwhere r is the radius of the orbit having principal quantum number = n∵   a3a1=91  r  ∝  n2  here  a3  =  radius of  3rd  orbit∴       λ  =2π ×  9a13=6πa1