Whenever the same input can produce 2 or more different outputs (i.e. not unique), it is not a function. If every input produces a single output, then it is a function.
Note that in a function different inputs could produce the same value of output and remains a function, as long as EVERY input produces a SINGLE output.
Example: for the relation f(x)
{(1,1),(2,2).(3,1).(4,0)}
Each input produces a single output, so f(x) is a function.
for the relation g(x)={(1,1),(1,2).(2,1),(2,3)}
We see input of 1 has an output of either 1 or 2, so it is no longer a SINGLE output, so g(x) is not a function.
In Marco's case, each input produces a single output. So it is a function.
In Shelia's case, it is almost the same, except that an input of 3 could produce 3 or 6 as output, so it produces more than one output for the same input. So Shelia's case is not a function.