8. Calculate $$\sqrt{8} \cdot 21^2 \cdot \sqrt{8} \cdot 5^4$$.

Let's calculate this step-by-step:

1. The expression is $$\sqrt{8} \cdot 21^2 \cdot \sqrt{8} \cdot 5^4$$.
2. Rearrange the terms to group similar ones: $$(\sqrt{8} \cdot \sqrt{8}) \cdot 21^2 \cdot 5^4$$.
3. Calculate $$\sqrt{8} \cdot \sqrt{8}$$. This is simply 8.
4. Calculate $$21^2$$: $$21 \times 21 = 441$$.
5. Calculate $$5^4$$: $$5 \times 5 \times 5 \times 5 = 25 \times 25 = 625$$.
6. Now, multiply all the results together: $$8 \times 441 \times 625$$.
7. Let's multiply $$8 \times 625$$ first: $$8 \times 625 = 5000$$.
8. Now, multiply $$5000 \times 441$$.
9. $$5000 \times 441 = 5 \times 1000 \times 441 = 5 \times 441000$$.
10. $$5 \times 441000 = 2205000$$.

Ответ: 2,205,000