I am getting "expected nested-name-specifier" when I use "using"

I have an assignment that I have to use this filter.h file to apply a filter to an image but I can't even begin because for some reason the text of the code we are supposed to use doesn't work. The issue seems to lie in the using phrase as it keeps giving me an error "expected nested-name-specifier"

#ifndef FILTER_H_INCLUDED
#define FILTER_H_INCLUDED
#include <iostream>
#include <vector>
#include "ppm_utils.h"

using namespace std;
using Matrix = vector<vector<int>>;

namespace Filter {
  extern Matrix K3, K5;
  Image& sharpen(Image&, Matrix&);
  pixel_t apply_kernel(Image&, int x, int y, Matrix&);
  int clamp(int, int, int);
}




#endif // FILTER_H_INCLUDED