Question

Is {(-2,1),(-1,2),(0,3),(1,4) } a function?

Answer 1

Think of it this way, a function must pass the vertical line test.  There may not be 2 outputs for one input for example  (0, 1) (0, 2)  However, you can have something like this (1, 0) (2, 0)

This is indeed a function as it meets those criterion.  There are no two outputs for one input.