[SOLVED] Exact value of a Square's diagonal

This was a question on a Math test that I got wrong, I'm guessing because I didn't do proper exact value:

If the area of a square is 121 square inches, what is the exact value of one of the diagonals?

I used square roots to find length and width (11 inches), and Pythagorean theorem to find the diagonal (15.55634919 inches) but then forgot how to use exact value.

Can someone tell me if my previous math was correct, and then show me how to put the diagonals into exact value?