Answer:
 11 miles
Step-by-step explanation:
The length of the route is the sum of the lengths of the parts of the route.
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Let b represent the distance to the beach from the library.
 total distance = (home to library) +(library to beach)
 18 miles = 7 miles + b
 11 miles = b . . . . . . . . . . subtract 7 miles from both sides of the equation
The distance from the library to the beach is 11 miles.
