since we want that CB and BF be perpendicular, then the measure of angle CBF must be 90, then, we have the following equation:
[tex]6x+18=90[/tex]
solving for x, we get:
[tex]\begin{gathered} 6x+18=90 \\ \Rightarrow6x=90-18=72 \\ \Rightarrow x=\frac{72}{6}=12 \\ x=12 \end{gathered}[/tex]
therefore, x = 12
