How many inches is the box's side?
And please explain how you found the answer
Thank you :)

Step-by-step explanation:

I'm assuming 24' is the diagonal of the box (red dashed line) and not of the bottom face (black dotted line).

Option 1: easiest if you know/remember the formula for the distance of two points in space (or for the diagonal of a parallelepiped): where replacing (and squaring both sides to get rid of the square root) we get

Option 2: look at the triangle that has sides the forward left edge of the box, the dotted diagonal and the dashed diagonal. It's a right triangle of hypotenuse 24 and side length 12. Pythagorean theorem to find the missing side lenght

At this point we're back at the right triangle of the bottom face of the box, of hypotenuse 20.78' and side lenght 16'. Phythagorean theorem again: