Please help I don't understand!

Answer: SAS similarity theorem
Build proportional relation:
[tex]\Rightarrow \sf \dfrac{AC}{BC} = \dfrac{EC}{DC}[/tex]
[tex]\Rightarrow \sf \dfrac{200}{150} = \dfrac{320}{240}[/tex]
[tex]\Rightarrow \sf \dfrac{4}{3} = \dfrac{4}{3}[/tex]
Theorem followed: SAS similarity theorem