Let's simplify the expression:

$3(-2a - 4) + 3a$

Step 1: Distribute the $3$ over the terms inside the parentheses.

$3(-2a - 4) = 3 \times -2a + 3 \times -4 = -6a - 12$

Step 2: Combine the results.

$-6a - 12 + 3a$

Step 3: Combine like terms.

$-6a + 3a - 12 = -3a - 12$

Final result:

$-3a - 12$

This is the simplified form of the expression.

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Would you like more details or have any questions about the steps? Here are some related questions you might find useful:

1. How do you identify like terms in algebraic expressions?
2. What is the role of the distributive property in simplifying expressions?
3. How would you solve for $a$ if the expression $-3a - 12 = 0$?
4. What happens when you combine terms with different coefficients and variables?
5. Can this expression be factored in any way?

Tip: Always ensure that the signs are handled correctly during distribution, especially when multiplying with negative numbers!