OpenGL.h

#ifndef _OPENGL_H
#define _OPENGL_H
class OpenGL
{
public:
	bool SetupWindow(const char *title, int width, int height, int bits, bool isFullscreen);
	bool DisplayScene();
};
#endif

main.cpp

#include OpenGL.h

...

void HelloWorld()
{
	SetupWindow("OpenGL Interface",500,500,16,false);
	DisplayScene();
}

Errors :

1>.\Client\main.cpp(107) : error C3861: 'SetupWindow': identifier not found
1>.\Client\main.cpp(108) : error C3861: 'DisplayScene': identifier not found