In ΔPMC and ΔPXC,
∠PMC=∠PXC (perpendiculars)
∠PCM=∠PCX ( bisectors of \angle DCB)
CP = CP(common side)
∴ΔPMC≅ΔPXC (by AAS congruence)
Hence by cpct
PM = PX ........ (1)
Now in ΔPNB and ΔPXB,
∠PNB=∠PXB (perpendiculars)
∠PBN=∠PBX (bisectors of ∠CBA)
BP = BP(common side)
∴ΔPNB≅ΔPXB (by AAS congruency)
Hence by cpct
PN = PX........ (2)
∴ From (1) and (2) PM = PN
Hence P is equidistant from the opposite sides AB and CD