Appendix A - Review - A.2 Geometry Essentials - A.2 Assess Your Understanding - Page A20: 42

$A=30$, $B=75$, $C=75$ and $x=8$.

Since the triangles are similar, then the angles are equal. That is $A=30$, $B=75$, $C=75$. To find $x$, we set up a ratio between corresponding sides: $$ \frac{x}{16}=\frac{6}{12}=\frac{1}{2}\Longrightarrow x=\frac{16}{2}=8 . $$