\[5x^{2} + 8x - 4 = 0\]
\[x^{2} + \frac{8}{5}x - \frac{4}{5} = 0\]
\[x_{1} + x_{2} = - \frac{8}{5} = - 1,6.\]
\[Ответ:\ - 1,6.\]