Resolver el sistema de ecuaciones:
$$ \begin{cases} x^2 - xy + y^2 = 3 & \text{(I)} \\ 2x^2 - xy - y^2 = 5 & \text{(II)} \end{cases} $$

$$ \begin{array}{lll} \text{(a) } x = -\frac{1}{\sqrt{7}}, y = \frac{4}{\sqrt{7}} & \text{(b) } x = \frac{1}{\sqrt{7}}, y = \frac{4}{\sqrt{7}} & \text{(c) } x = \frac{4}{\sqrt{7}}, y = -\frac{1}{\sqrt{7}} \\ \text{(d) } x = -\frac{1}{\sqrt{7}}, y = \frac{4}{\sqrt{7}} & \text{(e) } x = -\frac{4}{\sqrt{7}}, y = \frac{1}{\sqrt{7}} & \end{array} $$