Đáp án:
$\begin{array}{l}
A = \left( {\sqrt {28} - \sqrt {12} - \sqrt 7 } \right).\sqrt 7 + 2{\sqrt {\left( {2\sqrt 7 - 3} \right)} ^2}\\
= 2\sqrt 7 .\sqrt 7 - 2\sqrt 3 .\sqrt 7 - \sqrt 7 .\sqrt 7 + 2.\left( {2\sqrt 7 - 3} \right)\\
= 14 - 2.\sqrt {21} - 7 + 4\sqrt 7 - 6\\
= 1 - 2\sqrt {21} + 4\sqrt 7
\end{array}$