Teorema da bissetriz interna :
[tex]\displaystyle \frac{\text{AB}}{\text{BD}}=\frac{\text{AC}}{\text{CD}}[/tex]
substituindo os respectivos valores :
[tex]\displaystyle \frac{18}{11-\text x}=\frac{15}{\text x}[/tex]
[tex]\displaystyle \frac{6}{11-\text x}=\frac{5}{\text x}[/tex]
[tex]6\text x = 5(11-\text x)[/tex]
[tex]6\text x = 55 - 5\text x[/tex]
[tex]11\text x = 55[/tex]
[tex]\huge\boxed{\text x = 5}\checkmark[/tex]
