Explanation:
the negation of
I am not drawing a rectangle
is
I am not not drawing a rectangle.
and that means
I am drawing a rectangle.
now, let's go through the statements, if they are equal to this :
I am drawing a circle is not the same as I am drawing a rectangle. so, no negation.
it is not the case that I am drawing a circle could mean that I am drawing a rectangle, but it could mean that I am drawing something else (besides a circle and a rectangle). so, it is not the same as the negated statement, and therefore, no negation.
I am drawing a rectangle is precisely the negated statement, and therefore yes, a negation.
I am not drawing a circle is the same as the second case. so, no negation.