To find $f(x) + g(x)$, we need to add the expressions for $f(x)$ and $g(x)$.

Given: $f(x) = 2x + 1$ $g(x) = 3x$

Now, add the two functions: $f(x) + g(x) = (2x + 1) + 3x$

Combine like terms: $f(x) + g(x) = 2x + 3x + 1 = 5x + 1$

So, the correct answer is:

D. $5x + 1$

Would you like any further details or have any other questions? Here are 5 related questions to expand your understanding:

1. What is $f(x) - g(x)$ for the given functions?
2. How would you find $f(g(x))$ for the given functions?
3. What is the value of $f(x) + g(x)$ when $x = 2$?
4. Can you find $f(x) \times g(x)$ for these functions?
5. What is the inverse function of $f(x)$ and $g(x)$?

Tip: When combining functions, always group and simplify like terms to make the expression more manageable.