Solution
= Solution
The diagonal <Cartan subalgebra> consists of
$$
\operatorname{diag}(t_1,t_2,t_3,-t_1,-t_2,-t_3).
$$
Let $\varepsilon_i\in\mathfrak t^*$ read off $t_i$. The roots are
$$
\boxed{
\Phi=\{\pm\varepsilon_i\pm\varepsilon_j:
1\leq i<j\leq3\}},
$$
the <D3 root system>.