what is the input if the output is 0 ?

In other words, what is y if x is 0.
Look on the graph, a line passes through point, [tex]A(x,y)\longrightarrow A(0,-3)[/tex]
So we can conclude that at that very point the input x was 0 and the output y was -3 therefore,
[tex]\boxed{f(-3)=0}[/tex]
Hope this helps.
r3t40