Let's visualize this problem (See the picture below). The red line represents the diagonal that is 14 cm, the green sides represent the 25-cm sides, and the yellow line represents the diagonal that we need to find. From this, you can see that the Pythagorean Theorem can be applied to this problem (Since the diagonals are perpendicular)
a^2+b^2=c^2
14^2+b^2=25^2
196+b^2=625
b^2=429
b=20.71 (approximate)
:)
