This is an algorithm to multiplying two matrix

mult_matrix.c

#include <stdio.h>

int main() {
  int matriz[3][3], matriz1[3][3] = {{1, 2, 3}, {2, 3, 1}, {3, 2, 1}}, matriz2[3][3]={{1, 2, 3}, {2, 3, 1}, {3, 2, 1}};

  for (int k = 0; k < 3; k++) {
    for (int i = 0; i < 3; i++) {
      int aux = 0;
      for (int j = 0; j < 3; j++) {
        aux += matriz1[k][j] * matriz2[j][i];
      }
      matriz[k][i] = aux;
      aux = 0;
    }
  }

  for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
      printf("%d ", matriz[i][j]);
    }
    printf("\n");
  }
  
  return 0;
}