\[при\ \ b = - 5,\ \ \]
\[- 15x - 1 = 0,\ один\ корень\]
\[= 9b^{2} - 12b - 60 > 0\]
\[9b^{2} - 12b - 60 = 0\ \ |\ :3\]
\[3b^{2} - 4b - 20 = 0\]
\[D = 16 + 240 = 256\]
\[b_{1} = \frac{4 - 16}{6} = - 2\]
\[b_{2} = \frac{4 + 16}{6} = 3\frac{1}{3}\]