To model the distance traveled, \( y \), based on the hours driven, \( x \), after they have already traveled 500 miles, the equation would be:
\[ y = 70x + 500 \]
This indicates that for each hour driven at an average speed of 70 miles per hour, the family will add that distance to the initial 500 miles they have already traveled.
This situation is nonproportional because the equation includes a constant term (500), which means the distance does not start at zero.
So the correct response is:
y = 70x + 500, nonproportional.