Answer:
To reflect a triangle across the y-axis, you need to negate the x-coordinate of each vertex.
The original triangle with vertices (1, 3), (2, 5), and (4, 3) will be reflected across the y-axis to become a new triangle with the following vertices: (-1, 3), (-2, 5), and (-4, 3).
Keep in mind that a reflection across the y-axis means that the object is flipped along the y-axis. Therefore, the x-coordinate of each vertex is multiplied by -1, which flips the object horizontally.