So, I understand the formula to achieve the diagonal of a square is a variation of Pythagoras' Theorem, which is the square root of 2 times the length squared.

My exact side length is 2 + 2√3. So I input this length in the formula and I get an answer of 4√2, however, in the book the answer is 4√2+√3, with the √3 being under the first radical.

I cannot see how they are getting to this answer, any help would be fantastic.

Thanks in advance.