There exists a unique pair of polynomials $P(x)$ and $Q(x)$ such that:
\[ \begin{align*} P(Q(x)) &= P(x)(x^2-6x+7) \\ Q(P(x)) &= Q(x)(x^2-3x-2) \end{align*} \]
Compute $P(10) + Q(-10)$.

Let's think step by step and output the final answer within \boxed{}.
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