6. This identification is given by the Killing form $\kappa$ on the Lie algebra $\mathfrak{g}$. For each $\lambda \in \mathfrak{h}^*$, there exists a unique $t_\lambda \in \mathfrak{h}$ such that $\lambda(h) = \kappa(t_\lambda, h)$ for all $h \in \mathfrak{h}$.
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