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

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:

$$3a - 3ab - C + bC$$

From the first group, $3a - 3ab$, the common factor is $3a$.

$$3a(1 - b)$$

From the second group, $-C + bC$, the common factor is $-C$.

$$-C(1 - b)$$

Now the expression is:

$$3a(1 - b) - C(1 - b)$$