\(\left\{{}\begin{matrix}C\left(x;y\right)\\C\left(a;\dfrac{9}{4}\right)\end{matrix}\right.\Rightarrow y=\dfrac{9}{4}\)
\(y=6x\Rightarrow x=\dfrac{y}{6}=\dfrac{\dfrac{9}{4}}{6}=\dfrac{3}{8}\)
\(\left\{{}\begin{matrix}C\left(a;\dfrac{9}{4}\right)\\C\left(\dfrac{3}{8};\dfrac{9}{4}\right)\end{matrix}\right.\Rightarrow a=\dfrac{3}{8}\)