Ответ:

  1. \((\sqrt{6})^2=6\).
  2. \((-\sqrt{21})^2=21\).
  3. \((-4\sqrt{5})^2=(-4)^2(\sqrt{5})^2=16\cdot5=80\).
  4. \(\left(\frac{\sqrt{6}}{3}\right)^2=\frac{6}{9}=\frac{2}{3}\).

Ответ: 1) 6; 2) 21; 3) 80; 4) \(\frac{2}{3}\).