Decidability of $SEQ_{CFG} = \{⟨G,H⟩ \mid \text{$G,H$ are CFGs and $L(G) ⊆ L(H)$}\}$
We also know that $L(G)=L(H)$ iff both $L(G)\subseteq L(H)$ and $L(H)\subseteq L(G)$.

Now assume $SEQ_{CFG}$ is decidable, then we can decide both ...