Actually, it is quite simple.
Let's say the distance from Seattle to the wilderness is X.
Then, based on two-way trip totally spend 6 hours and known each one way average velocity, we can get this equation which is X/132 + X/110=6
Solve this simple equation, X*(1/132+1/110)=6
242X=6*132*110
So, X=360
The answer is 360 miles.