$14.$ A mixture containing $12\mathrm{.}8$ uM analyte $($X$)$ and $444$ of internal standard $($IS$)$ gave a ning an chromatographic peak area of $306$ for X and $511$ for IS$.$ A second solution contai unknown quantity of X plus $55\mathrm{.}5$ of IS had peak areas of $251$ for X and $563$ for ls$.$ Find the concentration of X in the second solution. $(3)$