Where it says calculate I didnt quite understand what it wants so id did the following;
Sqrt(2^2+1^2 = Sqrt(5)
Argument(2-i)= atan(-1/2)= -26.6
In polar form, 2-i is: Sqrt(5), theta= 333.4
As you have been told already, convert to the polar form which Plato did in post 2, then use DeMoivre's Theorem, like you have been instructed.