/in/foo.c: In function 'main':
/in/foo.c:4:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
  printf("Entrer deux equation\n");
  ^~~~~~
/in/foo.c:4:2: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:4:2: note: include '<stdio.h>' or provide a declaration of 'printf'
/in/foo.c:7:3: warning: implicit declaration of function 'scanf_s' [-Wimplicit-function-declaration]
   scanf_s("%f", &a[i][j]);
   ^~~~~~~
/tmp/ccFp8WWA.o: In function `main':
foo.c:(.text.startup+0x20): undefined reference to `scanf_s'
foo.c:(.text.startup+0x33): undefined reference to `scanf_s'
foo.c:(.text.startup+0x46): undefined reference to `scanf_s'
foo.c:(.text.startup+0x59): undefined reference to `scanf_s'
foo.c:(.text.startup+0x6c): undefined reference to `scanf_s'
/tmp/ccFp8WWA.o:foo.c:(.text.startup+0x7f): more undefined references to `scanf_s' follow
collect2: error: ld returned 1 exit status