Approximately 7.81 miles apart.
If you were to draw lines between the three points, you would see that it makes a triangle, with leg lengths 6 and 5, like in the attached file.
To find the distance between Hillsboro and Middletown, or the hypotenuse of the triangle, use the Pythagorean Theorem:
6² + 5² = c²
c² = 61
c = √61 = 7.81 miles
Hope this helps!