Mathematics for IGCSE & O level - Transformations (Section 6 - No. 5)
When a shape is reflected over the y-axis, the coordinates transform by:
(x, y) -> (-x, y)
(x, y) -> (x, -y)
(x, y) -> (-x, -y)
(x, y) -> (y, x)
Paliwanag
Reflecting across the y-axis changes (x, y) to (-x, y).