\[x² + mx + 9\]
\[m = 6:\]
\[x^{2} + 6x + 9 = (x + 3)^{2}.\]
\[m = - 6:\]
\[x^{2} - 6x + 9 = (x - 3)^{2}.\]
\[Ответ:m = \pm 6.\]