Answer: diagonal \(= 5\sqrt{2}\) cm \(\approx 7.07\) cm
Each diagonal of a square of side \(a\) is \(a\sqrt{2}\) (by the Pythagoras theorem in the right triangle formed by two sides and the diagonal).
Diagonal \(= 5\sqrt{2} = 5 \times 1.414 \approx 7.07\) cm.