Well try testing smaller numbers to see why you can't just subtract.
Let's try 2^3-2^2. Expanding out both terms gives you 8-4=4. Now let's factor this to see if that can make some sense. 2^3-2^2=2^2(2-1)=2(1)=2
Apply this same technique to your problem.
2^29-2^28=2^28(2-1)=2^28(1)=2^28
And divide by two. So (2^28)/2=2^27