To find $f(a + h)$ for the function $f(x) = 5x^3$, you simply substitute $x = a + h$ into the function. Here's how you do it:

$f(x) = 5x^3$

Now, substitute $x = a + h$:

$f(a + h) = 5(a + h)^3$

So, $f(a + h)$ is:

$f(a + h) = 5(a + h)^3$

Would you like to expand $(a + h)^3$ for further simplification? Let me know if you'd like more details or have any questions.

Here are 5 related questions you could explore:

1. How do you expand $(a + h)^3$?
2. What is the derivative of $5x^3$?
3. How would you compute the limit of $f(a + h)$ as $h \to 0$?
4. How does the function $f(x) = 5x^3$ behave as $x$ increases?
5. What is the difference between a function and its shifted version $f(a + h)$?

Tip: When substituting values into functions, always make sure to carefully replace all instances of $x$ with the new expression!