The question asks:

Jika 3, x, dan 9 merupakan tiga suku pertama barisan geometri, tentukan nilai x.

This means: "If 3, $x$, and 9 are the first three terms of a geometric sequence, determine the value of $x$."

In a geometric sequence, the ratio between consecutive terms is constant. Let's denote the common ratio by $r$. Thus, the following relationships hold:

$\frac{x}{3} = r \quad \text{and} \quad \frac{9}{x} = r$

Since both equal $r$, we can set these two equations equal to each other:

$\frac{x}{3} = \frac{9}{x}$

Now, let's solve this equation for $x$:

$x^2 = 3 \times 9$ $x^2 = 27$ $x = \sqrt{27} = 3\sqrt{3}$

Therefore, the value of $x$ is $3\sqrt{3}$.

Would you like more details or have any questions about this? Here are some related problems you could explore:

1. What is the common ratio $r$ in this geometric sequence?
2. Can you find the next term in this geometric sequence?
3. How do you generalize the formula for the $n$-th term in a geometric sequence?
4. What is the sum of the first $n$ terms of a geometric sequence?
5. How would the solution change if the geometric sequence was decreasing?

Tip: Always check if the terms in a geometric sequence follow the correct ratio between consecutive terms to avoid calculation mistakes.