If ${\mathrm{e}}_1$ and ${\mathrm{e}}_2$ are the eccentricites of the ellipse $\frac{{\mathrm{x}}^2}{18}+\frac{{\mathrm{y}}^2}{4}=1$ and the hyperbola $\frac{{\mathrm{x}}^2}{9}-\frac{{\mathrm{y}}^2}{4}=1$ respectively and $({\mathrm{e}}_1 , {\mathrm{e}}_2 )$ is a point on the ellipse $15{\mathrm{x}}^2 + 3{\mathrm{y}}^2 = \mathrm{k}$, then k is equal to $\mathrm{\_\_\_\_}$