Your Universe is D, set B ={swimmers}, set C = {weightlifters}.
First, you are looking for what is either swimmers or weightlifters and that is given by the union of sets B and C: B∪C
Then, you have to look for everything that is not swimmers or weightlifters, and that is the complement of the previous set: ¬(B∪C)
An other notation could be everything that is in the Universe which is not a swimmer or a weightlifter: D - (B∪C)
The area will be therefore the one outside the circles.