`a) 19 - 6\sqrt{2}`

`= 18 - 2 . 3\sqrt{2} . 1 + 1`

`= ( 3\sqrt{2} )^2 - 2 . 3\sqrt{2} . 1 + 1^2`

`= ( 3\sqrt{2} - 1 )^2`

`b) 11 - 6\sqrt{2}`

`= 9 - 2 . 3 . \sqrt{2} + 2`

`= 3^2 - 2 . 3 . \sqrt{2} +( \sqrt{2})^2`

`= ( 3 - \sqrt{2 } )^2`

`c) 9 - 6\sqrt{2}`

`= 6 - 2 . \sqrt{3} . \sqrt{2} . \sqrt{3} + 3`

`= ( \sqrt{6})^2 - 2 . \sqrt{6} . \sqrt{3} + ( \sqrt{3} )^2`

`= ( \sqrt{6} - \sqrt{3} )^2`