Line segments AD and DC are congruent. We know this because of the two slash marks that are on the line segments.
Knowing that they are congruent, we can determine that they are equal to one another.
AD = DC
y + 6 = 2y
Subtract y from both sides of the equation.
6 = y
Knowing that 6 is y, solve for both AD and DC.
AD = 6 + 6 = 12
DC = 2(6) = 12
Solution: DC is 12
