To solve for the missing value \( x \) in the equation \( 6x = 9 \), we need to understand the units involved. Here, the left-hand side of the equation involves speed (miles per hour), and the right-hand side involves distance (miles).
In the equation \( 6x = 9 \), if we let \( x \) represent the time in hours, we can rearrange the equation to find \( x \):
\[ x = \frac{9}{6} = 1.5 \text{ hours} \]
Thus, the missing value \( x \) represents time, and the unit for \( x \) should be hours.