Here's one way of doing this
Let the starting point = (0,0)
So 18 mi north puts us at (0,18)
11 miles west of here puts us at (-11,18)
6 miles south puts us at ( -11, 12)
6 miles east puts us at (-5 , 12)
These are two legs of a 5, 12, 13 right triangle
So...we are 13 miles from our starting point
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