To solve this problem, consider the following picture
By the tangent secant segment theorem we have the following equation
[tex]a^2=b\cdot(b+c)[/tex]
Note that in our case, we have a=x, b=3 and c=9. So if we replace this values in the equation, we have
[tex]x^2=3\cdot(3+9)=3\cdot12=36[/tex]
so, applying the square root on both sides, we get
[tex]x=\sqrt[]{36}=\pm6[/tex]
Since x is a distance, it should be strictly positive, so we have that
[tex]x=6[/tex]
