- $ M = \\\left\\\{ {x \\\in \\\left. \\\mathbb{R} \\\right|{x^2} - 6x + 9 = 0} \\\right\\\}$.
- $ M = \\\left\\\{ {x \\\in \\\left. \\\mathbb{Z} \\\right|{x^2} = 0} \\\right\\\}$.
- $ M = \\\left\\\{ {x \\\in \\\left. \\\mathbb{Q} \\\right|3x + 2 = 0} \\\right\\\}$.
- $ M = \\\left\\\{ {x \\\in \\\left. \\\mathbb{N} \\\right|2x - 1 = 0} \\\right\\\}$.