When you get 7 as the 2nd figure in the square root of 256 you can check if it is right, as we did in the video, by comparing the square of that 7 with the 16 at the end of 256.
Since 16 and 49 are not the same we can go back and, instead of saying 15/2 = 7 remainder 1, we can say 15/2 = 6 remainder 3.
Doing that you then see that you have 36 at the end of your 256 now and that is the same as 6 squared.
So the answer is 16. Writing 6 rem 3 instead of 7 rem 1 is something we will be using in lesson 25 next week, so you have anticipated that.
The full square root process comes up in lesson 29 where the above process is extended.