If the height of transmitting and receiving antennas are 80m each,...

Given: $R = 6 . 4 \times {10}^{6}m$

Using the relation between the range of the radio communication and the height of the transmission antenna ${h}_{T}$ and the receiving antenna ${h}_{R}$, the maximum line of sight distance between two antennas, is {d}_\max = \sqrt{2R{h}_{T}} + \sqrt{2R{h}_{R}}

⇒ {d}_\max = 2 \times \sqrt{2 \times 6 . 4 \times {10}^{6} \times 80} = 64000m = 64km