Since P atoms are conserved, applying POAC for P atoms,
moles of P in NaH2PO4 = moles of P in Mg2P2O7
1 x moles of NaH2PO4 = 2 x moles of Mg2P2O7
(1 mole of NaH2PO4 contains 1 mole of P and 1 mole of Mg2P2O7 contains 2 moles of P)
Wt. of NaH2PO4 = 1.14 g.