You can locate the construction site for the library by using the Circumcenter Theorem, which states that the circumcenter (the library) of a triangle is equidistant from the vertices (three towns) of the triangle.
To solve questions of these sorts, bring all the terms to one side & simplify it leaving x alone in the other side of the equation (other side of the equal to sign). After simplification you'll get the value of x.
Remember that the signs & operations will change when you bring a term to the other side of the equation.
So, addition will become subtraction, subtraction will become addition, multiplication will become division & division will become multiplication.