Answer:
33
Explanation:
The triangle naming tells us the corresponding (and congruent) sides are ...
We can use this last congruence to find the length of side DF.
CA ≅ FE
2x +5 = 21 . . . . . . substitute the given values for CA, FE
4x + 10 = 42 . . . . multiply by 2
4x + 1 = 33 . . . . . subtract 9. This is the length of DF
Side DF is 33 units in length.
_____
We could solve for x (=8) and then substitute that into the expression for DF. It is easier just to go there directly, since we can.