To solve the equation \( 123 - 23 \cdot (42 - x) = 100 \), we can follow these steps:

1. **Distribute the -23**:
\[
123 - 23 \cdot 42 + 23x = 100
\]

2. **Calculate \( 23 \cdot 42 \)**:
\[
23 \cdot 42 = 966
\]
So the equation becomes:
\[
123 - 966 + 23x = 100
\]

3. **Combine the constant terms**:
\[
-843 + 23x = 100
\]

4. **Add 843 to both sides**:
\[
23x = 100 + 843
\]
\[
23x = 943
\]

5. **Divide by 23 to solve for \( x \)**:
\[
x = \frac{943}{23}
\]

6. **Calculate \( 943 \div 23 \)**:
\[
x = 41
\]

So the solution is:
\[
\boxed{41}
\]