Answer:
[tex]\huge\boxed{x=\sqrt{21}}[/tex]
Step-by-step explanation:
ΔABD and ΔBCD are similar (AAA).
Therefore the corresponding sides are in proportion:
[tex]\dfrac{BD}{CD}=\dfrac{AD}{BD}[/tex]
Substitute
[tex]BD=x;\ CD=3;\ AD=7[/tex]
[tex]\dfrac{x}{3}=\dfrac{7}{x}[/tex] cross multiply
[tex](x)(x)=(3)(7)\\\\x^2=21\to x=\sqrt{21}[/tex]
