`đặt` `b,A=sqrt{12-3sqrt{7}}-sqrt{12+3sqrt{7}}`
`sqrt{2}A=sqrt{24-6sqrt{7}}-sqrt{24+6sqrt{7}}`
`sqrt{2}A=sqrt{21-2.sqrt{21}.sqrt{3}+3}-sqrt{21+2.sqrt{21}.sqrt{3}+3}`
`sqrt{2}A=sqrt{(sqrt{21}-sqrt{3})^2}-sqrt{(sqrt{21}+sqrt{3})^2}`
`sqrt{2}A=sqrt{21}-sqrt{3}-sqrt{3}-sqrt{21}=-2sqrt{3}`
`=>A=-sqrt{6}`