gcc error message: stray '\XXX' in program

I've started learning Assembly, and I have two files:

main.c
#include <stdio.h>
int assembly(void);
int main(void)
{
    printf("Resultado: %d\n", assembly());
return 0;
}

and

assembly.asm
global assembly
assembly:
  mov eax, 777
  ret

So I used this command "nasm assembly.asm -f elf64" and it created the file "assembly.o". After that, I used another command "gcc -c main.c -o main.o" but it showed some errors on the shell:

What can I do?