So:

int？ n = 2；

n+= n-= n * n；

Equivalent to:

n-= n * n；

n+= n；

Then it is equivalent to:

n = n-n * n；

n = n+n；

Because n is initialized to 2:

n = n-n * n = 2-2 * 2 =-2；

n = n+n =-2+(-2)= =-4；

So the final result is -4.