The marginal distribution for gender tells you the probability that a randomly selected person taken from this sample is either male or female, regardless of their blood type.
In this case, we have total sample size of 714 people. Of these, 379 are male and 335 are female. Then the marginal probability mass function would be
where G is a random variable taking on one of two values (male or female).
I did the math! basically, i did 168-156, and got 12. Then i added, 168+12, and got 180. So i know that both angles together make 180 degrees, which is the definition of supplementary angles.