C ++ error LNK2019: unresolved external symbol "

#ifndef ELEMENTAL_H
#define ELEMENTAL_H

#include "SDL_init.h"
#include "Sprite.h"


class Elemental: public Game::Sprite{
public: 
	Elemental( SDL_Surface* img, int x, int y );

	//virtual ~Sprite();			// Destructor.
	void tick();					// Loop which controls and updates a specific sprite object with the background objects.
	
	SDL_Surface* get_image();

	double getSpeed_X();			// Will get the speed ( x ) of the object
	double getSpeed_Y();			// Will get the speed ( y ) of the object
	
	void set_speed( double x, double y );	// Will set the speed ( x ) of the object
	
	void move(); // Will be in main class
	void show(); // Will be in main class

	void install_sprite( std::string filename );
	void delete_sprite();
	
protected:

private:
	double speed_x, speed_y, x_pos, y_pos;
	SDL_Surface* sprite_image;
	SDL_Rect ebox;
	
};

#endif

/*********************************************************
This is the root class for all gameobjects.

**********************************************************/
#ifndef SPRITE_H
#define SPRITE_H

#include "SDL_init.h"
#include <string>

const int SPRITE_WIDTH = 50;
const int SPRITE_HEIGHT = 56;

namespace Game
{
class Sprite
{
public:
	Sprite();
	Sprite( std::string filename );				// Constructor.
	Sprite::Sprite( SDL_Surface* img );

	//virtual ~Sprite();		// Destructor.
	virtual void tick();		// Loop which controls and updates a specific sprite object with the background objects.
	SDL_Surface* get_image();

	double getPos_X();				// Will get the x position of the object
	double getPos_Y();				// Will get the y position of the object
	double getSpeed_X();			// Will get the speed ( x ) of the object
	double getSpeed_Y();			// Will get the speed ( y ) of the object
	
	void move_sprite(double x, double y );	// Moves the sprite object to a specific position.
	void move();
	void show();
	void set_pos( double x, double y );// Will set the position ( x ) of the object
	void set_speed( double x, double y );	// Will set the speed ( x ) of the object

	void install_sprite( /*Sprite* installed,*/ std::string filename );
	void delete_sprite( /*Sprite* deleted */);
	
	//Sprite* getInstance();			// Will create a neW sprite object and place it on the screen.
	

protected:
	//Sprite() {};

private:
	double speed_x, speed_y, x_pos, y_pos;
	SDL_Surface* sprite_image;
	SDL_Rect box;
	bool jumped;
};

}
#endif