the third set represent a function
For the points to represent a function then for each value of x there can only be one corresponding value of y
sets 1, 2 and 4 do not obey this and have more than one value of y for a given x
set 1
1 → 1, 2, 3, 4, 5
set 2
3 → 3, - 3 and 5 → 5, - 5
set 4
- 1 → - 1, 1 and - 2 → 4, - 4
