Start by showing that triangle ABC is simuilar to both triangle ADE and triangle BEF, hence ADE is similar to BEF. This means that AD/EF = DE/BF. Finally since EF=DC, and DE=CF, you're there.
Start by showing that triangle ABC is simuilar to both triangle ADE and triangle BEF, hence ADE is similar to BEF. This means that AD/EF = DE/BF. Finally since EF=DC, and DE=CF, you're there.