If the BD bisector is ∠ABC, then BD is the angular bisector of ∠ABC, and any point on the angular bisector is equal to the distance between the two sides of the corner. Therefore, if point D is taken as DE where point E is perpendicular to AB, there must be DE=EC=3.

If you haven't studied the bisector theorem, you can use congruent triangles to do it. According to the corner theorem, it can be obtained that all triangles DCB are equal to triangles DEB, and DE=EC=3.