Does anybody know this? I don't get it.

scale factor = 1/2

Step-by-step explanation:

for AC: 4 + 4 = 8

8 is the AC of the red triangle

4 is the AC of the blue triangle

to get from 8 to 4, we divide by 2 or multiply by 1/2 (they mean the same thing)

10 is the CB of the red triangle

5 is the CB of the blue triangle

to get from 10 to 5, we divide by 2 or multiply by 1/2