I'm not sure on how to do this problem may someone and explain this to me? in advance : )

The line of symetry divides a shape into 2 equal parts that match.

Only line P bisects the triangle in such a way that both sides would be identical.

Hence, P is correct.

A line of symmetry is a line that could perfectly cut a shape in half, for example, if you have a piece of paper and you fold it in half that crease would be like of symmetry, However, the shape must perfectly overlap itself, meaning that there are no extra shapes around the edgesbtw you have the correct answer.
{hope I helped... :) }