if the total energy of the first excited state of hydrogen atom is taken to be zero then the potential energy of the ground state of the hydrogen atom will be?( Ionisation energy of ground state H-atom is 13.6)

Dear student,

Taking the energy of an undbound electron to be zero, the energy of first excited state and ground state were -3.4eV and -13.6eV.
Hence the potential energy of the electron is -6.8eV and kinetic energy is 3.4eV. But in the new system, the potential energy has been chosen such that the total energy is zero. Hence the potential has been increased by 3.4eV to -3.4eV, so that the total energy is -3.4eV+3.4eV=0eV.

Therefore the potential energy of the electron in ground state would also increase by 3.4eV and become -27.2eV+3.4eV=-23.8eV