Barnet

1. It's simple algebra really.

2. Keys divisible by 3: 3, 6 or 9.

3. Key that contains itself and another key when squared: 5 (25) or 6 (36).

4. Run through all the possible math as presented in the clue. 3 x 5 = 15. 6 x 5 = 30. 9 x 5 = 45.

5. None of those work with the clue. 6 x 6 = 36. Nope. 9 x 6 = 54. Still no. What's left?

(SOLUTION FOLLOWS)

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6. Choose 3 and 6 for the first two slots. 3 + 6 = 1 + 8. 3 x 6 = 18. 3618 is your answer.