The set of all \( a \in \mathbb{R} \) for which the equation \( x|x-1| + |x+2| + a = 0 \) has exactly one real root, is:
\( (-6, -3) \)
\( (-\infty, -3) \)
\( (-6, \infty) \)
\( (-\infty, \infty) \)