a)
n*m=m*n=(5-\sqrt3)(2\sqrt3+1)=10\sqrt3+5-6-\sqrt3=9\sqrt3-1
b)
\frac{m}{n}=\frac{5-\sqrt3}{2\sqrt3+1}*\frac{2\sqrt3-1}{2\sqrt3-1}=\frac{10\sqrt3-5-6+\sqrt3}{(2\sqrt3)^2-1^2}=
=\frac{11\sqrt3-11}{11}=\sqrt3-1
c)
|m-n|=|5-\sqrt3-2\sqrt3-1|=|\-3\sqrt3+4|-(-3\sqrt3+4)=3\sqrt3-4