Not sure how to go on about finding this constraint. The constraint asks for; either both or neither of $x_1$ and $x_2$ should appear.
What I have so far is that y can be either binary values $0$ or $1$. When the value is $0$, then neither $x_1$ nor $x_2$ appear. When the value is $1$, $x_1$ and $x_2$ must both appear (they must be one or greater).
When $y=0$, then $x_1+x_2=0$. When $y=1$, then $x_1\times x_2\geq 1$. The issue is I'm having trouble incorporating the y into these constraints.