If $e_1$ and $e_2$ are the eccentricitis of the ellipse, $\frac{x^2}{18}+\frac{y^2}{4}=1 \quad$ and the hyperbola, $\quad \frac{x^2}{9}-\frac{y^2}{4}=1$ respectively and ( $e_1, e_2$ ) is a point on the ellipse, $15 x^2+3 y^2=k$, then $k$ is equal to
