Since the triangles are similar thie means that:
[tex]\frac{8}{x}=\frac{12}{21}[/tex]
Solving for x we have:
[tex]\begin{gathered} \frac{8}{x}=\frac{12}{21} \\ 8=\frac{12}{21}x \\ x=\frac{8}{\frac{12}{21}} \\ x=\frac{21\cdot8}{12} \\ x=\frac{168}{12} \\ x=14 \end{gathered}[/tex]
Therefore x=14
