Is your question about determining the contrapositive when the necessary condition is already negative? If so, you negate the negative! That is, not x becomes not not x, and thus x.
An example: If the specimen is a fish, it is NOT a mammal.
To find the contrapositive, we negate the negative. If it is a mammal, the specimen is not a fish.
If you encounter this on a practice problem please comment there, and I’ll help show you how it applies with respect to that particular question.
