`a)`
`\sqrt{11} + 4 \sqrt{7} - \sqrt{11} - 4 \sqrt{7}`
` = (\sqrt{11} - \sqrt{11}) + (4\sqrt{7} - 4\sqrt{7})`
` = 0 + 0`
` = 0`
`b)`
`\sqrt{19} - 8 \sqrt{3} + \sqrt{19} + 8\sqrt{3}`
` = (\sqrt{19} + \sqrt{19}) + ( 8\sqrt{3} - 8\sqrt{3})`
`= 2 \sqrt{19}`
`c)`
`\sqrt{9} + 4 \sqrt{2} + \sqrt{9} - 4 \sqrt{2}`
` = ( \sqrt{9} + \sqrt{9}) + (4 \sqrt{2} - 4 \sqrt{2})`
` = 2 . \sqrt{9}`
` = 2 . 3 = 6`
`d)`
`\sqrt{28} - 6 \sqrt{3} - \sqrt{28} + 6 \sqrt{3}`
` = (\sqrt{28} -\sqrt{28}) + (6 \sqrt{3} - 6\sqrt{3})`
`= 0 + 0`
`= 0`
`e)`
`\sqrt{21} - 12 \sqrt{3} - \sqrt{21} + 12 \sqrt{3}`
` = (\sqrt{21} - \sqrt{21}) + (12 \sqrt{3} - 12 \sqrt{3})`
` = 0 + 0 `
` = 0`
`f)`
`\sqrt{29} + 12 \sqrt{5} - \sqrt{29} - 12 \sqrt{5}`
`= (\sqrt{29} - \sqrt{29}) + (12\sqrt{5} - 12\sqrt{5})`
` = 0 + 0 `
` = 0`
`g)`
`\sqrt{5} + 2 \sqrt{6} - \sqrt{5} + 2 \sqrt{6}`
`= (\sqrt{5} - \sqrt{5}) + (2\sqrt{6} + 2 \sqrt{6})`
` = 4 \sqrt{6}`
`h)`
`\sqrt{8} - 2 \sqrt{15} + \sqrt{8} + 2 \sqrt{15}`
` = (\sqrt{8} + \sqrt{8}) + (2\sqrt{15} - 2 \sqrt{15})`
`= 2 \sqrt{8}`
` = 2 . \sqrt{4} . \sqrt{2}`
` = 2 . 2 . \sqrt{2} = 4\sqrt{2}`
