Question 792455
The side of the square you cut will be the height of the box.

you cut a square of side x
so area of aquare = x^2

you cut 4 squares 
so area of 4 squares = 4x^2

The total area of the cardboard = 100 * 100 sq, cm

Area required for the box = total area - area of the cut squares

100^2-4x^2 =6400

4x^2=10000-6400

4x^2=3600
/4
x^2= 900

x= 30 taking the positive value