To find the value of $3a + 4$ when $a = 5$, substitute $a$ with 5:

$3a + 4 = 3(5) + 4$

$= 15 + 4$

$= 19$

Thus, the value of $3a + 4$ is 19.

Would you like a breakdown of the steps or an explanation of related concepts? Here are some questions to consider:

1. What is substitution in algebra, and why is it useful?
2. How can similar expressions like $4a - 7$ be solved?
3. What happens if $a$ is a negative number or a fraction?
4. Can this be visualized on a graph? If yes, how?
5. What does the term $3a + 4$ represent in a real-world context?

Tip: Always double-check your substitution to avoid small mistakes in calculations.