$\begin{equation*} \begin{cases} x^{2}_{1} + x^{2}_{2} = 65 &\\ x_1 + x_2 = 3 &\\ x_1x_2 = a & \end{cases} \end{equation*}$
$(x_1 + x_2)^2 = 3^2$
$(x_1 + x_2)^2 = 9$
$x^{2}_{1} + 2x_1x_2 + x^{2}_{2} = 9$
$(x^{2}_{1} + x^{2}_{2}) + 2x_1x_2 = 9$
$65 + 2a = 9$
$2a = 9 - 65$
2a = −56
a = −28
Ответ: −28