If `triangle ABC cong triangle DEF` then
`CA` = ________

Answer

To solve the problem, we need to determine the length of side CA in triangle ABC given that triangle ABC is congruent to triangle DEF.

### Step-by-Step Solution:

1. **Understand the Congruence**: We are given that triangle ABC is congruent to triangle DEF. This means that all corresponding sides and angles of the two triangles are equal.

2. **Identify Corresponding Parts**: In congruent triangles, the corresponding parts are equal. The notation for congruence can be represented as:
- Triangle ABC ≅ Triangle DEF

From this, we can identify the corresponding sides:
- Side AB corresponds to side DE
- Side BC corresponds to side EF
- Side CA corresponds to side FD

3. **Apply CPCT**: CPCT stands for Corresponding Parts of Congruent Triangles. According to CPCT, since triangle ABC is congruent to triangle DEF, we can state:
- AB = DE
- BC = EF
- CA = FD

4. **Conclusion**: Since we need to find the length of CA, we can conclude that:
- CA = FD

Thus, the answer is:
**CA = FD**