How do I debug a new class in arduino IDE

I’ve decided that I want to create some classes to make my life easier. The first one that I wanted to create was a “Line”. I get an error message and want to figure out how i can debug this?, this is my code:

#include <Line.h>

Line line();

void setup() {
  // put your setup code here, to run once:
  line.set(1, 2, 3, 4);

void loop() {
  // put your main code here, to run repeatedly: 

This is my error message:
ColourSphereCode.cpp: In function 'void setup()':
ColourSphereCode:6: error: request for member 'set' in 'line', which is of non-class type 'Line ()()'
ColourSphereCode.cpp: In function 'void loop()':
ColourSphereCode:11: error: request for member 'm' in 'line', which is of non-class type 'Line ()()'

And this is my Line.h:
#ifndef Line_h
#define Line_h

#include "Arduino.h"

class Line
    void set(double x1,double  y1,double  x2,double y2);
  double m();
  double c();
  double _x1;
  double _y1;
  double _x2;
  double _y2;


and this is my Line.cpp:

#include "Arduino.h"
#include "Line.h"


// define the two end points of the line
void Line::set(double x1, double y1, double x2, double y2)
  _x1 = x1;
  _y1 = y1;
  _x2 = x2;
  _y2 = y2;

// return the slope of the line
double m()
  return 1.1;

// return the offset of the line
double c()
  return 2.3;

so it would be great if someone can see where I’m going wrong but it would be even better if there is an 'easy" way that I can learn how to debug these errors and work with developing classes. The morse example looks a little thin on the ground for this, but maybe there is another place that I can look?

Moderator edit:
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tags added.

Line line();

This declares a function, taking no arguments, called "line," returning a Line.
You want to declare a global variable initialized by the default (no-argument) constructor:

Line line;

And, yes, I've done C++ since version 1 (25 years ago? something like that) and this still bites me once in a while...