summation and your other functions are defined after they're used in main, and so the compiler has made a guess about it's signature; in other words, an implicit declaration has been assumed.
You should declare the function before it's used and get rid of the warning. In the C99 specification, this is an error.
Either move the function bodies before main, or include method signatures before main, e.g.:
#include <stdio.h>
int summation(int *, int *, int *);
int main()
{
// ...
