I’d personally say 4, but it could be 2.
6^2 is 36 and 7^2 is 49. That puts 43 7 numbers from 6^2 and 6 numbers away from 7^2. 6 is obviously smaller than 7 making 7^2 the obvious choice, but I don’t know if you’re allowed to go over. I did sqrt(43) in my calculator and it gave me 6.5574... depending on how you want to round, I’d again say 4
