For a square all the sides are the same.
So all the sides are 10 cm as stated by the question.
If the square is cut diagonal, it will be a right triangle.
Let x cm be the diagonal of a square.
10² + 10² = x² (Pythagorean Theorem)
100 + 100 = x²
x² = 200
x = √200
x = 10√2 or 14.14 (nearest hundredths)