Answer: (2.1/3) [if I read the numbers correctly]
Step-by-step explanation:
Similar triangles have identical angles, desite having different sides. But the sides remain in the same proportions. I can't quite make out the exact numbers since the photo is too small, but it the lower tiangle has sides of 2.1 and 3, the second triangle will have the same two sides in the same proportion: 2.1/3.
