Given: 3 commerce books, 3 computer books, and 5 economics books.

To find: The number of ways in which books of the same subject are come together.

Approach:

- We can group the books based on the subject to simplify the problem.
- We can consider three groups: commerce books, computer books, and economics books.
- Now, we can arrange these three groups in a row in different ways.
- Within each group, we can also arrange the books in different ways.
- Finally, we can find the total number of arrangements by multiplying the number of ways each step can be done.

Solution:

Step 1: Group the books based on the subject.

- Group 1: 3 commerce books.
- Group 2: 3 computer books.
- Group 3: 5 economics books.

Step 2: Arrange the groups in a row.

- We can arrange the three groups in a row in 3! = 6 ways.

Step 3: Arrange the books within each group.

- Within Group 1 (commerce books), we can arrange the books in 3! = 6 ways.
- Within Group 2 (computer books), we can arrange the books in 3! = 6 ways.
- Within Group 3 (economics books), we can arrange the books in 5! = 120 ways.

Step 4: Find the total number of arrangements.

- The total number of arrangements is given by the product of the number of ways each step can be done.
- Total number of arrangements = 6 × 6 × 120 = 4,320.

Therefore, the number of ways in which books of the same subject are come together is 4,320.

However, this is not the final answer as we need to consider the arrangements in which books of the same subject are placed together. We can treat each group as a single entity and arrange the three entities in a row in 3! ways. Within each group, we can arrange the books in 3! × 3! × 5! ways. Hence, the total number of arrangements in which books of the same subject are placed together is given by:

3! × 3! × 5! = 6 × 6 × 120 = 4,320

We need to subtract this from the total number of arrangements to get the final answer:

Total number of arrangements = 11!/(3! × 3! × 5!) = 25,920

Final answer = Total number of arrangements - Arrangements in which books of the same subject are placed together

Final answer = 25,920 - 4,320 = 21,600

Hence, the correct option is (c) 21,600.