Let $\mathrm{H}_{\mathrm{n}}: \frac{x^{2}}{1+n}-\frac{y^{2}}{3+n}=1, n \in N$. Let $\mathrm{k}$ be the smallest even value of $\mathrm{n}$ such that the eccentricity of $\mathrm{H}_{\mathrm{k}}$ is a rational number. If $l$ is the length of the latus rectum of $\mathrm{H}_{\mathrm{k}}$, then $21 l$ is equal to ____________.