$$6x^2 + 3x = a$$, при $$x = -0.5$$
$$6 \cdot (-0.5)^2 + 3 \cdot (-0.5) = a$$
$$6 \cdot 0.25 - 1.5 = a$$
$$1.5 - 1.5 = a$$
$$a = 0$$
$$6x^2 + 3x = 0$$
$$3x(2x + 1) = 0$$
$$x_1 = 0$$, $$x_2 = -\frac{1}{2}$$
Ответ: a = 0, x = 0