The P-P bond energy is  'x' KJ/mole.  Then the energy needed for the dissociation of 124 g of white phosphorous is

Atomic weights: P=31
124 g p means 124/31 p atoms.
i,e. , 4 p atoms.

so there are 6 P-P bonds.

For breaking 1 p-p bond, we need 'x' kj/mol.

Therefore, for breaking of 6 p-p bond, 

we require $6 \times \text{'}x\text{'} kj/mol$ energy.