Ответ:

  1. \((2-\sqrt{a})(2+\sqrt{a})-4=4-a-4=-a\).
  2. \((3-\sqrt{2a})(3+\sqrt{2a})+4a=9-2a+4a=9+2a\).
  3. \((2\sqrt{3}-3\sqrt{a})(2\sqrt{3}+3\sqrt{a})=12-9a\).
  4. \((2\sqrt{7}-\sqrt{7a})(2\sqrt{7}+\sqrt{7a})=28-7a\).