A quadratic has a double root when the discriminant \(b^2-4ac\) is equal to \(0\).
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Rearraging the equation grants \(x^2+bx+9=0 \Longrightarrow\) \(a = 1, b = b, c = 9\)
\(b^2 - 4(1)(9) = 0\)
\(b^2 = 36\\ b = \boxed{\pm 6}\)
