Look at the picture.
ΔZYX and ΔDYC are similar. Therefore the lengths of sides are in proportion:
[tex]\dfrac{DC}{ZX}=\dfrac{DY}{ZY}[/tex]
DC = x
ZX = 16
DY = a
ZY = a + a = 2a
Substitute:
[tex]\dfrac{x}{16}=\dfrac{a}{2a}[/tex]
[tex]\dfrac{x}{16}=\dfrac{1}{2}[/tex] multiply both sides by 16
[tex]\boxed{x=8}[/tex]
