To find $f(g(6))$, we need to follow these steps:

1. Find $g(6)$:

The function $g(x) = x^2 + 4$. Substitute $x = 6$ into the equation:

$g(6) = 6^2 + 4 = 36 + 4 = 40$

2. Use this result to find $f(g(6))$, which is $f(40)$:

The function $f(x) = 5x - 1$. Now substitute $x = 40$ into the equation:

$f(40) = 5(40) - 1 = 200 - 1 = 199$

Therefore, $f(g(6)) = 199$.

Would you like any additional details or clarifications?

Here are 5 related questions for further practice:

1. What is $g(f(3))$ given the same functions $f(x)$ and $g(x)$?
2. Find the composition $f(g(x))$ in terms of $x$.
3. What is the inverse of the function $f(x) = 5x - 1$?
4. Calculate $g(g(5))$ for the given function $g(x)$.
5. If $h(x) = f(x) + g(x)$, what is $h(6)$?

Tip: When solving compositions of functions, always work from the inside out, evaluating the innermost function first.