The bottom of the cone is a circle, and the bottom of the cube is a square with variable length.
You can draw on paper and cut out the largest circle on a square, which should be a circle with a radius of a/2.
The maximum height of a cone with a cube at the bottom is the side length a of the cube.
Therefore, the side length of cube A, the radius of cone bottom circle a/2, and the height of cone A.
Just calculate according to the cone volume formula.
Note that the issuer does not need to fully open the value of a, and it may disappear in the future.