Ok so first let me list out all operators used here..
- ()
- +=
- ,
Now above operator list I wrote, it is as per C's precedence rule here
So first, it let's take () part,
(a+=3,5,a) => As we know, comma proceeds in left-to-right, so this is equal to (a=4,5,a),
Now from above we can say a is having value 4 and we have used parenthesis here, it will take last value which is not but a so finally
a+= a
=> a+=4
=> a=a+4
and we know a=4 hence
=>a = 4+4=8
so answer is 8.