Answer:
3 units
Step-by-step explanation:
We can see that triangle ADE is similar to triangle ACB.
Therefore the ratios of the lengths of their sides must be equal.
We can write AD/AC = AE/AB
we are given AD = 9, AE = 12 and AB = 12 + 4 = 16
Substituting the above values into the equation:
AD/AC = AE/AB
9 / AC = 12/16 (rearranging)
(AC)(12) = (9)(16)
AC = (9)(16)/12
AC = 12
We can also see that
DC = AC - AD
DC = 12 - 9
DC = 3 units
