Is this a function [1,-5) (2,-4) (1,-4) ]

No, it is not a function because 1 has 2 outputs. For example two X values, 3 & 4, can have the same value for Y, (3,2) & (4,2) but the X value can only have one Y values

{ (1,-5) (2,-4) (1,-4) } Not a function

{ (1,-5) (2,-4) (3,-5) } Is a function