### Video Transcript

Triangle π΅πΈπΉ is the image of triangle π΅π·πΉ by reflection across what.

To start, letβs 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, letβs 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 π΅π·πΉ is the object thatβs being reflected. And therefore, our missing word or description must be the line of reflection. So weβre told that triangle π΅πΈπΉ is the image of triangle π΅π·πΉ. We can see that point πΈ is a reflection of point π·. And we can see that point πΉ is the image of itself, and point π΅ 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 πΉπ΅. So we have triangle π΅πΈπΉ is the image of triangle π΅π·πΉ by reflection across the line πΉπ΅.