`a)` `\sqrt{15-6\sqrt{6}}+\sqrt{33-12\sqrt{6}}`
`=\sqrt{9-2.3\sqrt{6}+6}+\sqrt{24-12\sqrt{6}+9}`
`=\sqrt{(3-\sqrt{6})^2}+\sqrt{(2\sqrt{6}-3)^2}`
`=|3-\sqrt{6}|+|2\sqrt{6}-3|`
`=3-\sqrt{6}+2\sqrt{6}-3`
`=\sqrt{6}`
`b)` `\sqrt{11+4\sqrt{6}}+\sqrt{9-4\sqrt{2}}`
`=\sqrt{8+4\sqrt{6}+3}+\sqrt{8-4\sqrt{2}+1}`
`=\sqrt{(2\sqrt{2}+\sqrt{3})^2}+\sqrt{(2\sqrt{2}-1)^2}`
`=|2\sqrt{2}+\sqrt{3}|+|2\sqrt{2}-1|`
`=2\sqrt{2}+\sqrt{3}+2\sqrt{2}-1`
`=4\sqrt{2}+\sqrt{3}-1`
`c)` `\sqrt{7-2\sqrt{10}}-\sqrt{6-2\sqrt{5}}`
`=\sqrt{5-2\sqrt{5.2}+2}-\sqrt{5-2\sqrt{5}+1}`
`=\sqrt{(\sqrt{5}-\sqrt{2})^2}-\sqrt{(\sqrt{5}-1)^2}`
`=|\sqrt{5}-\sqrt{2}|-|\sqrt{5}-1|`
`=\sqrt{5}-\sqrt{2}-\sqrt{5}+1`
`=1-\sqrt{2}`
