How the code executes?

Question

#include <stdio.h>

int sum_up (int x, int y);

int main() {
int x, y, result;

x = 3;
y = 12;
result = sum_up(x, y);
printf("%d + %d\ = %d\n", x, y, result);

return 0;
}

int sum_up (int a, int b) {
a+= b;
return(a);
}

As I declared formal variables as x and y but I defined a and b then also how it's working?

Please explain me, how the code execute at machine level