ΔADC and ΔCDB are similar. Therefore the sides are in proportion:
[tex]\dfrac{AD}{DC}=\dfrac{DC}{DB}[/tex]
Substitute:
[tex]\dfrac{x}{6}=\dfrac{6}{4}[/tex] cross multiply
[tex]4x=(6)(6)[/tex]
[tex]4x=36[/tex] divide both sides by 4
[tex]\boxed{x=9}[/tex]
