Question

how i guess the perpendicular side of the cube

Answer 1

I don't follow your question. If it is a cube, every side is perpendicular to its adjoining sides.

Answer 2

Please start over. Where did this problem come from? Have you shared every bit of information that came with the problem (such as the instructions and a diagram)?
Why would you want to "guess" the length of the "perpendicular side of the cube?"
In math we don't "guess;" rather, we "estimate," and then (hopefully) calculate the desired quantity to a reasonable degree of accuracy.