What’s the minimal lenghth of each side of an equilateral triangle that can get a square of 15 mm inside it?

If I want to know What’s the minimal length of each side of an equilateral triangle that can get a square of which each of its sides is 15 mm, inside it (=inside the square)?. How to calculate it?
(I’m not looking for the answer, but I’m looking for the way or approach to solving it, for future self-solutions).