### Video Transcript

Triangle ๐ต๐ธ๐น is the image of triangle ๐ต๐ท๐น by reflection across what.

To start, letโs think what possible thing we could be writing for our missing value. Is it, for example, a shape or a line or a point? Well, letโs recall that if we wanted to describe a reflection, we would first need to state what object is being reflected. And then, we would need to say the line of reflection.

In our question, triangle ๐ต๐ท๐น is the object thatโs being reflected. And therefore, our missing word or description must be the line of reflection. So weโre told that triangle ๐ต๐ธ๐น is the image of triangle ๐ต๐ท๐น. We can see that point ๐ธ is a reflection of point ๐ท. And we can see that point ๐น is the image of itself, and point ๐ต is also the image of itself.

This means that both these two points must lie on the line of reflection. Therefore, our line of reflection is the line ๐น๐ต. So we have triangle ๐ต๐ธ๐น is the image of triangle ๐ต๐ท๐น by reflection across the line ๐น๐ต.