[latex]2 sqrt{7}* sqrt{21}=2 sqrt{7*7*3}=2*7sqrt{3} =14sqrt{3} \ \ (2 sqrt{2}-sqrt{50} )*sqrt{2}=(2 sqrt{2}-sqrt{25*2} )*sqrt{2}=(2 sqrt{2}-5sqrt{2} )*sqrt{2}= \ =(-3 sqrt{2})*sqrt{2}=-3*2=-6 \ \ frac{4 sqrt{80} - sqrt{125}}{ sqrt{5}} =frac{4 sqrt{16*5} - sqrt{25*5}}{ sqrt{5}} =frac{4*4 sqrt{5} -5 sqrt{5}}{ sqrt{5}} =frac{16sqrt{5} -5 sqrt{5}}{ sqrt{5}} =frac{11sqrt{5}}{ sqrt{5}} =11[/latex]