$\begin{equation*} \begin{cases} x^2_{1} - x^2_{2} = \frac{1}{4} &\\ x_1 + x_2 = -\frac{5}{2} &\\ x_1x_2 = \frac{c}{2} & \end{cases} \end{equation*}$
$\frac{x^2_{1} - x^2_{2}}{x_1 + x_2} = x_1 - x_2 = \frac{1}{4} * (-\frac{5}{2}) = -\frac{1}{10}$
$\begin{equation*} \begin{cases} x_1 - x_2 = \frac{1}{10} &\\ x_1 + x_2 = -\frac{5}{2} &\\ x_1x_2 = \frac{c}{2} & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} 2x_1 = -\frac{5}{2} + \frac{1}{10} = -2,4 &\\ 2x_2 = -\frac{5}{2} - \frac{1}{10} = -2,6 &\\ 2x_1x_2 = c & \end{cases} \end{equation*}$
$2x_1x_2 = 2(-1,2)(-1,3) = 3,12$
c = 3,12
Ответ: 3,12