Explanation:
step 1. a function must only have a single y value for each x value.
step 2. for the input x, there can only be one output or result y.
step 3. #1 is a function because for each x there is only one y.
step 4. #2 and #3 are functions also.
step 5. #4 is not a function because (1, 1) and (1, -1) says x is 1 but the y is both 1 and -1 (2 values)
step 6. (2, 2) and (2, -2) also makes #4 not a function.