\[14x² + 25x - 84 = 0\]
\[D = 625 + 4704 = 5329 = 73²\]
\[x_{1,2} = \frac{- 25 \pm 73}{28}\]
\[x_{1} = - \frac{12}{7} = - 1\frac{5}{7};\ \ \ x_{2} = 3,5\]
\[Ответ:3,5;\ \ - 1\frac{5}{7}.\]