Given:
PO is the perpendicular bisector of triangle MON.

1. To find the value of x:
Since,

Therefore, the angle measure of OPN is 90 degrees.
So that,

Hence the value of x is 16.
2. To find MN:
Since PO is the perpendicular bisector to the side MN.
So, MP=PN
Therefore,

Hence, the length of MN is 74.