bi(two)...sector(section). Bisector cuts the segment into two equal sections.
BC = CD definition of perpendicular bisector
BC + CD = BD segment addition postulate
CD + CD = BD substitution
2CD = BD added like terms
2CD = 16 substitution
CD = 8 division property of equality
CD = y + 3 given per graph
y + 3 = 8 transitive property
y = 5 subtraction property
Answer: 5