In geometry, it is always advantageous to draw a diagram from the given information in order to visualize the problem in the context of the given.
A figure has been drawn to define the vertices and intersections.
The given lengths are also noted.
From the properties of a kite, the diagonals intersect at right angles, resulting in four right triangles.
Since we know two of the sides of each of the right triangles, we can calculate their heights which in turn are the segments which make up the other diagonal.
From triangle A F G, we use Pythagoras theorem to find
h1=A F=sqrt(20*20-12*12)=sqrt(256)=16
From triangle DFG, we use Pythagoras theorem to find
h2=DF=sqrt(13*13-12*12)=sqrt(25) = 5
So the length of the other diagonal equals 16+5=21 cm
