Use the same argument names while defining the function.
Use int mul (int a, int b) instead of int mul (int x, int y)
you just missed the library "#include <stdio.h>"
int mul (int a, int b);main(){ int a, b, c; a = 5; b = 10; c = mul (a,b); printf ("multipilcation of %d and %d is %d",a,b,c);} int mul (int x, int y)
{ int p; p = x*y; return(p); }