Item 6 Find the value of $p$ .

A triangle is drawn. One leg is labeled 12, one leg is labeled 24, and one leg is labeled 21. A segment is drawn from the vertex between the side labeled 12 and the side labeled 24 to the side labeled 21, splitting it into 2 segments, the longer of which is labeled p and is adjacent to the side labeled 24. The two angles formed at the vertex split by the line to the third side are marked as congruent.