Geometry

Question Analysis:
This question tests the understanding of the equal - side property of equilateral triangles and basic algebraic skills.
The main focus is on using the fact that all sides of an equilateral triangle are equal.

Key Concept Explanation:
In an equilateral triangle, all sides are equal.
So if two sides are given as 3x and 9, we can set up an equation based on this property.

Step - by - Step Solution:
Since all sides of an equilateral triangle are equal, we set up the equation 3x = 9.
Dividing both sides by 3, we get x = 3.

Option Analysis:
Option A: If x = 1, then 3x = 3, which is not equal to 9, so this is incorrect.
Option B: If x = 2, then 3x = 6, which is not equal to 9, so this is incorrect.
Option C: If x = 3, then 3x = 9, which satisfies...