**False**

The statement "23 - (-21) = 23 + 21" is false. Let's break it down to understand why.

**Explanation:**
- To simplify the expression 23 - (-21), we need to understand the concept of subtracting a negative number. Subtracting a negative number is the same as adding its positive counterpart.
- In this case, subtracting -21 is the same as adding 21. So, 23 - (-21) becomes 23 + 21.
- When we add 23 and 21, we get 44. Therefore, the correct equation is 23 - (-21) = 44, not 23 + 21.
- The mistake in the given statement is that it incorrectly assumes that subtracting a negative number results in subtracting the two numbers. However, it should be adding the positive counterpart of the negative number.

In summary, the correct statement should be "23 - (-21) = 44" instead of "23 - (-21) = 23 + 21."

True