Cho \(A = \left\{ {x \in \mathbb{R}\,\,\left| {\left| {mx - 3} \right| = mx - 3} \right.} \right\}\), \(B = \left\{ {x \in \mathbb{R}\,\,\left| {{x^2} - 4 = 0} \right.} \right\}\). Tìm \(m\) để \(B\backslash A = B\).