1. The problem asks for the length of the altitude $n$ of an equilateral triangle with side length 6. 2. In an equilateral triangle, all sides are equal, and the altitude splits the triangle into two 30-60-90 right triangles. 3. The sides of a 30-60-90 triangle have a fixed ratio: the side opposite 30° is $x$, opposite 60° is $x\sqrt{3}$, and the hypotenuse is $2x$. 4. Here, the hypotenuse of the right triangle is the side of the equilateral triangle, which is 6. 5. So, $2x = 6 \implies x = 3$. 6. The altitude $n$ is opposite the 60° angle, so $n = x\sqrt{3} = 3\sqrt{3}$. 7. Therefore, the length of the altitude is $3\sqrt{3}$. Answer: D. $3\sqrt{3}$