#include <iostream>
using namespace std;
typedef unsigned short int USHORT;
USHORT xvalue, yvalue;
USHORT doublex (int, int); // function prototype
int main (){
	cout << "Please enter your value for x\n";
	cin >> xvalue;
	doublex (xvalue, yvalue);
	cout << yvalue;
	return yvalue;
}

USHORT doublex (USHORT xvalue)
{
	USHORT yvalue;
	yvalue = (xvalue * 2);
	return yvalue;

}

compiling gives no errors, when linking i get 

Linking...
Cpp1.obj : error LNK2001: unresolved external symbol "unsigned short __cdecl doublex(int,int)" (?doublex@@YAGHH@Z)
Debug/Cpp1.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.

Cpp1.exe - 2 error(s), 0 warning(s)