`2)`

`a)`

`(\sqrt{28}-2\sqrt{14}+\sqrt{7}).\sqrt{7}+7\sqrt{8}`

`=\sqrt{2^{2}.7.7}-2\sqrt{2.7.7}+\sqrt{7^{2}}+7\sqrt{2^{2}.2}`

`=2.7-2.7\sqrt{2}+7+7.2\sqrt{2}`

`=14+7-14\sqrt{2}+14\sqrt{2}`

`=21`

`b)`

`D=(\sqrt{4/3}-\sqrt{3}+\sqrt{25/3}).\sqrt{12}`

`=\sqrt{4/3. 12}-\sqrt{3.12}+\sqrt{25/3. 12}`

`=\sqrt{16}-\sqrt{36}+\sqrt{100}`

`=4-6+10`

`=8`

Vậy `D=8`