\[2x^{2} - x - 15 = 0\]
\[D = 1 + 120 = 121\]
\[x_{1} = \frac{1 + 11}{4} = 3;\]
\[x_{2} = \frac{1 - 11}{4} = - \frac{10}{4} = - 2,5.\]