for an expression or relation to be a function, it must not have any x-coordinate values repeated, let's check this one
[tex]\bf (6,3), (\stackrel{\stackrel{rep eated}{\downarrow }}{-1},2), (\stackrel{\stackrel{rep eated}{\downarrow }}{-1},-1), (4,3)\qquad \textit{not a function}[/tex]
