Answer:
[tex]\huge\boxed{x=\sqrt{70}}[/tex]
Step-by-step explanation:
ΔABC and ΔADB are similar (AAA).
Therefore the corresponging sides are in proportion:
[tex]\dfrac{AB}{AD}=\dfrac{AC}{AB}[/tex]
Substitute:
[tex]AB=x\\AC=7+3=10\\AD=7[/tex]
[tex]\dfrac{x}{7}=\dfrac{10}{x}[/tex] cross multiply
[tex](x)(x)=(7)(10)\\\\x^2=70\to x=\sqrt{70}[/tex]
