Answer:
When we have a point (x, y) and we do a reflection over a given line, we know that the new point (x', y') will be at the same distance from the line as our initial point (x,y).
Now, in this case, we have a reflection over the line y = -1. (this line is parallel to the x-axis)
But in the image, we can see that the reflected triangle is drawn in the other side of the y-axis, this means that the reflection was made in a line parallel to the y-axis.
Then the mistake that Oscar did is that he reflected over the wrong line, seems that he reflected the triangle over the line x = -1 instead of the line y = -1.
