To reflect a triangle over the y-axis, you need to change the sign of the x-coordinates of its vertices.

When you perform a reflection of a triangle across the y-axis, you create a mirror image of the triangle on the opposite side of the y-axis. This process involves taking each vertex (corner) of the triangle and inverting its x-coordinate while keeping the y-coordinate unchanged.

For instance, consider a triangle with vertices $A(2, 3)$, $B(4, 5)$, and $C(6, 1)$. To reflect these points over the y-axis, you would change the x-coordinates to their negatives. Therefore, the reflected vertices would be $A'(-2, 3)$, $B'(-4, 5)$, and $C'(-6, 1)$.

This transformation can be expressed mathematically as $(x, y) \rightarrow (-x, y)$. By applying this rule to each vertex of the triangle, you ensure that the shape and size of the triangle remain unchanged, while its position is mirrored across the y-axis. This technique is particularly useful in various geometric problems and enhances your understanding of symmetry and transformations in coordinate geometry.