Finding the value of $xy+xz+6yz$

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If $x^2-2y=-13, y^2-4z= 14$, and $z^2+6x=-15$ then find the value of $xy+xz+6yz$.

Which way should I proceed. I did try to simplify the expressions but it was of no use. Please help.

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After summing our equations we obtain: $$x^2+y^2+z^2+6x-2y-4z+14=0$$ or $$(x+3)^2+(y-1)^2+(z-2)^2=0.$$ Can you end it now?