The line which the shape is exactly equal two halves is called line of symmetry. For the given pentagon the line of symmetry is line which divide pentagon in two equal halves and vertices of each side halves lie at equal distance from the line of symmetry.
So if we divide the pentagon by line x = 3 then it passes through one its vertices and divide the pentagon in two halves which are equal and simillar to each other.
So for pentagon line of symmetry is x = 3.
