THE EXPLANATION:
Let's say the box is sitting on one of the rectangular sides that measure 6 cm by 12 cm.
We call that the base of the box.
The diagonal splits that base into 2 right triangles with legs measuring 6 cm by 12 cm.
The length of that diagonal, cm, according to the Pythagorean theorem, is such that .
That diagonal is perpendicular to one of the vertical 4-cm long edges of the box.
The longest possible stick, of length cm, stick will form a right triangle with the diagonal of the base and a vertical edge.
This is what the (see through) box looks like:
According to the Pythagorean theorem,