Describe the recursive formula ?

Which of the following of the following sets of side lengths could not form a triangle?

The figure shows triangle def and line segment bc, which is parallel to ef: triangle def has a point b on side de and point c on side df. the line bc is parallel to the line ef. part a: is triangle def similar to triangle dbc? explain using what you know about triangle similarity. part b: which line segment on triangle dbc corresponds to line segment ef? explain your answer. part c: which angle on triangle dbc corresponds to angle f? explain your answer. asap

Solve for x: 5/3 x = 20/21 it's fraction. remember to simplify your fraction