Everyone here and elsewhere seems to be saying how simple this puzzle is but I don't think so. A lot of deduction and staring at numbers is involved, plus a knowledge of how square numbers work. I found a spreadsheet with all the square numbers on it useful as well as a list of prime numbers. And, xij, there are no mistakes in the clues - extra brackets are unnecessary.
I've finished it but I too am baffled by the ungrammatical title. And there seems to be no logic to the lettering. Why is one set A, B, C and not J, K, L? And Why those letters and not D, E, F etc?