Let's represent the unknown number as \( x \). According to the problem, the equation can be set up as follows:
\[ \frac{8 + x}{3} = 6 \]
Now, to solve for \( x \), we first multiply both sides by 3 to eliminate the fraction:
\[ 8 + x = 18 \]
Next, we isolate \( x \) by subtracting 8 from both sides:
\[ x = 18 - 8 \]
\[ x = 10 \]
So, the number is \( 10 \). However, since this option is not listed in your responses (6, -2/3, -6), I suggest double-checking the provided answers.