Well, you're given a Triangle MNO.
The length of MO is set and is Equal to PO.
|MO| = |PO|
You know that the sum of length of the smaller sides of a triangle is greater than the length of the largest side, and that the sum of the length of any two sides is greater than the third side.
Since you don't know the length of MN and ON, you have 3 situations
|MN| = |ON|, |MN| > |ON|, |MN| < |ON|
if |MN| = |ON|, then |x| + |MN| > |ON|
if |MN| > |ON|, then no matter what |x| you add, |x| + |MN| > |ON|
if |MN| < |ON|, you know that |MO| + |MN| > |NO|. Substitute |MO| for |PO|, then |PO| + |MN| > |NO|
I'll leave it up to you to construct a more formal proof.