Hi, it's me again.
The y-axis would be the line split vertically down the middle. That said, if you would have to reflect the figure over the y-axis, you would have to think of the line as a mirror. That said, if you're reflecting across the y axis to the from the right to left side, just change the numbers from (x,y) to (-x,y).
(4,6) => (-4,6)
(4,3) => (-4,3)
(6,3) => (-6,3)