To do this, we take half of the coefficient of the linear term, square it, and add it to both sides of the equation. For the x terms, we have:
$$
\frac{1}{2}(6) = 3
$$
So we add and subtract $3^2 = 9$ inside the parentheses:
$$
(x^2 + 6x + 9) + (y^2 - 2y) = -6
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