To find the constant of proportionality for Julian's walking, we need to determine his walking rate in miles per hour. From the information given, we know that it takes Julian 12 hours to walk 2 miles.
We can calculate his walking rate (constant of proportionality) using the formula:
\[ \text{Rate} = \frac{\text{Distance}}{\text{Time}} \]
Substituting in the distance and time:
\[ \text{Rate} = \frac{2 \text{ miles}}{12 \text{ hours}} = \frac{1}{6} \text{ miles per hour} \]
Thus, the constant of proportionality \( y \) in terms of \( x \) (where \( y \) corresponds to miles and \( x \) corresponds to hours) is:
\[ \frac{1}{6} \text{ miles per hour} \]
So, the constant of proportionality is \( \frac{1}{6} \) miles per hour.