Bohr's model is used to solve this question,
According to this E=−13.6×Z2n2
So, in this the E is equal to energy of electron in nth shell of atom whose atomic number is Z
.
Now, same amount of energy with opposite sign must be given to electron in order to remove it. I.E.=−13.6×Z2eV (As n=1)
Since for Lithium, Z=3
Therefore, you get I.E.=−122.4eV