I mean when C is negative

If $C$ is a negative number, let's represent it as $C = -k$, where $k$ is a positive number.
Substituting this into the equation:

$$x^2 + (-k) = 0$$ $$x^2 - k = 0$$

Therefore, the solutions for $x$ are real numbers.

For example, if $C = -4$:
$x^2 - 4 = 0$
$x^2 = 4$
$x = \pm \sqrt{4}$
$x = \pm 2$
These are real numbers.

The solutions will be real numbers.
$\text{Real numbers}$