in a proof, you could say that since AD is congruent to BD, since that is given to you, and angle ACD is congruent to angle CDB since a perpendicular bisector makes both sides a right angle, and CD is congruent to CD, (I forgot the reasoning for this part). after this, you can assume that triangle ACD is congruent to triangle CDB. and that would mean AC is congruent to CB because of CPCTC (Corresponding Parts of Congruent Triangles are Congruent), which would then mean that since AC is 13, CB would be 13 as well.
