Since the quadrilaterals are similar, then, corresponding sides of them are proportional. Then:
[tex]\frac{BC}{AB}=\frac{RS}{QR}[/tex]
Substitute the length of each segment into the equation and solve for x:
[tex]\begin{gathered} \Rightarrow\frac{x}{12}=\frac{4}{6} \\ \Rightarrow x=\frac{4}{6}\times12 \\ \Rightarrow x=8 \end{gathered}[/tex]
Therefore:
[tex]x=8[/tex]
