Factor the expression: 3a - c - 3ab + bc

Step 1: Group the terms with common factors. We can group the first two terms and the last two terms, or group terms with 'a' and terms with 'c'. Let's group terms that share $3a$ and terms that share $C$. $3a - C - 3ab + bC$ Rearrange the terms to group them more clearly: $3a - 3ab - C + bC$

Step 2: Factor out the common factor from each group. From the first group, $3a - 3ab$, the common factor is $3a$. $3a(1 - b)$ From the second group, $-C + bC$, the common factor is $-C$. $-C(1 - b)$

Step 3: Combine the factored groups. Now the expression is: $3a(1 - b) - C(1 - b)$

Step 4: Factor out the common binomial factor $(1 - b)$. $(1 - b)(3a - C)$

The factored expression is: $\boxed{(1 - b)(3a - C)}$