please help will give BRAINLIEST BRAINLIEST BRAINLIEST BRAINLIEST BRAINLIEST BRAINLIEST BRAINLIEST BRAINLIEST BRAINLIEST BRAINLIEST BRAINLIEST BRAINLIEST for correct answer!!!!!!!!!!!!!

Answer:
20
Step-by-step explanation:
Since both triangles are similar, then the ratio is 18:6, which is 3:1
Triangle ABC is 3 times the size of DEC.
So, 1/3 of 30 is 10, and 30-10 is 20.