Assume the following conditional statement is:
If 
It means:
If p, then q
The contrapositive is gotten by negative p and q
i.e.
If 
The above means:
If not p, then not q
Using the above analysis.
The conditional statement from the question is:
If a polygon is a parallelogram, then it is not a trapezoid.
The keywords to negate, which gives the contrapositive, are:
is a parallelogram
is not a trapezoid
When negated, the keywords become:
is not a parallelogram
is a trapezoid
So, the contrapositive is:
If a polygon is not a parallelogram, then it is a trapezoid.