Question

The point $(a,b)$ lies on the $y$-axis if $b=0$.

Answer 1

We know, in the $xy$-plane, any point on the $y$-axis will have $x=0$.
Hence, any point on the $y$-axis is of the form $(0,y)$, where $y$ can be any non-zero number.

Therefore, any point whose $x$-coordinate is $0$ and $y$-coordinate is non-zero will lie on the $y$-axis.

But here, if $b=0$ in the point $(a,b)$, then it clearly lies on the $x$-axis.

That is, the statement is false.

Therefore, $0$ is the answer.