The problem asks for $f(g(x))$, where:

• $f(x) = 6x - 3$
• $g(x) = 5x + 4$

To solve $f(g(x))$, we need to substitute $g(x)$ into $f(x)$.

Let's calculate step by step:

1. Start with $g(x) = 5x + 4$.
2. Now substitute $g(x)$ into $f(x)$: $f(g(x)) = f(5x + 4) = 6(5x + 4) - 3$
3. Expand and simplify: $f(5x + 4) = 6 \cdot 5x + 6 \cdot 4 - 3 = 30x + 24 - 3 = 30x + 21$

Thus, $f(g(x)) = 30x + 21$.

The correct answer is $\boxed{c}$.

Let me know if you want further details or have any questions! Here are some related questions for practice:

1. What is the general formula for the composition of two functions?
2. How do you find the inverse of a function?
3. If $h(x) = f(g(x))$, how would you differentiate $h(x)$?
4. How does the order of composition affect the result, i.e., is $f(g(x))$ the same as $g(f(x))$?
5. What happens to $f(g(x))$ if $g(x)$ is a constant function?

Tip: Always expand terms carefully when substituting functions to avoid algebraic errors.