We have to reflect the triangle across the y-axis.
If we have a point P = (x,y) and we reflect it across the y-axis, we will change the x-coordinate to its opposite value and and the y-coordinate is kept the same.
The image point then is P' = (-x,y).
Then, to reflect the triangle, we have to reflect the three vertices and then join the vertices.
We can then reflect the vertices as:
