a) `A=\sqrt{15-6\sqrt{6}}-\sqrt{33-12\sqrt{6}}`
`=\sqrt{6-6\sqrt{6}+9}-\sqrt{3^{2}-12\sqrt{6}+(2\sqrt{6})^{2}}`
`=\sqrt{(\sqrt{6}-3)^{2}}-\sqrt{(3-2\sqrt{6})^{2}}`
`=|\sqrt{6}-3|-|3-2\sqrt{6}|`
`=3-\sqrt{6}-2\sqrt{6}+3`
`=6-3\sqrt{6}`
b) `B=\sqrt{x^{2}-6x+9}+2x-1``(xle3)`
`=\sqrt{(x-3)^{2}}+2x-1`
`=|x-3|+2x-1`
`=3-x+2x-1`
`=2-x`
c) `C=12/(m-4)\sqrt{(m^{2}-8m+16)/9}``(m>4)`
`=12/(m-4).\sqrt{(m-4)^{2}}/\sqrt{3^{2}}`
`=12/(m-4).|m-4|/3`
`=12/(m-4).(m-4)/3`
`=4`
