Please explain the answer and how to get the answer?

50 people each did one activity at a sports centre.
Some of the people went swimming.
Some of the people played squash.
The rest of the people used the gym.
21 of the people were female.
6 of the 8 people who played squash were male.
18 of the people used the gym.
9 males went swimming.
Work out the number of females who used the gym.