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Sorry, I only know 14.
When you're completing the square, you have to half the x. -8x/2=-4. Therefore, this is what goes inside of the bracket. The bracket then becomes (x-4)^2, which when expanded, becomes x^2-4x-4x+16, which can then be simplified to x^2-8x+16 & in order to get just one at the end, you need to subtract the 15. Therefore, the completed square is (x-4)^2-15

Sorry it's only question, but hope this helps