code: public static double aForXAndH(double x, double h)
      {
          return MoreMath.solve(
                  (xx,a)->a*Math.cosh(xx/a)-a, 
                  x, 
                  h, 
                  Double.MIN_VALUE, 
                  100);
      }
docs: Returns a for a catenary having height h at x. Height is distance from
  vertex (a) to y.
  
  @param x
  @param h
  @return