\[x² - 3bx + 2b + 15 = 0\]
\[D = 9b^{2} - 8b - 20 > 0\]
\[9b^{2} - 8b - 20 = 0\]
\[D = 64 + 720 = 784\]
\[b_{1} = \frac{8 + 28}{18} = 2\]
\[b_{2} = \frac{8 - 28}{18} = - \frac{10}{9} = - 1\frac{1}{9}\]
\[\left( b + 1\frac{1}{9} \right)(b - 2) > 0\]
\[\]
\[Ответ:\left( - \infty;\ - 1\frac{1}{9} \right) \cup (2; + \infty).\]