how that work?

Question

#include <stdio.h>
#define x 10+5
int main() {
int a=x*x;
printf("%d",a);
return 0;
}

Answer 1

#define is preprocessor directive, and compiler replace all x in your code on 10+5, so your code for compiler will look like:
int a = 10 + 5 * 10 + 5;

And a = 65, because exist priority of operations

#define don't count, it replace

Answer 2

The expression

a = x * x;

Will be replaced with

a = 10 + 5 * 10 + 5;

Considering multiplication operation has higher priority over addition, multiplication of 5 * 10 is processed first, then the result (50) will be added by 10 to the left, and 5 to the right.