Which congruence statement says the same thing as triangle abc is congruent to triangle def? acb = dfe is the answer. but could you explain how these are the same?

When discussing similarity or congruence, the order of the letters in naming angles and polygons is important. There is a correspondence.

When you say ABC ≅ DEF, you are saying ...
   vertex A corresponds to vertex D
   vertex B corresponds to vertex E
   vertex C corresponds to vertex F

This will remain true for any order of the letters. If you name the vertices of the figures in reverse order, then you have ACB ≅ DFE. The above correspondences still hold.

This fact is handy because the correspondence remains when you name any subset of vertices (pairs, for example). The first and third letters of one name always correspond to the first and third letters of the other name, for example. You don't even need to see the figures; you only need to know their names.

21, i'm pretty sure it's right lol