` M = (4 + \sqrt{3}) \sqrt{19} - 8\sqrt{3} `
` <=> M = (4 + \sqrt{3}) \sqrt{4^2 - 2.4 \sqrt{3} + 3} `
` <=> M = (4 + \sqrt{3}) \sqrt{(4 - \sqrt{3})^2} `
` <=> M = (4 + \sqrt{3}).|4 - \sqrt{3}| `
` <=> M = (4 + \sqrt{3})(4 - \sqrt{3}) `
` <=> M = 16 - 3 `
` <=> M = 13 `