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Formula to square a retangle
The formula I can think of that you're looking for is the surface area formula for a cuboid (3 dimensional rectangle). The formula is; Area = 2lw+2wh+2hl, where l is the length, w is the width and h is the height. With the information you have, you can only find the area of one side of the slab, i.e 24 x 40 = 960 cm^2
Take the squared root of the product of two different sides of a rectangle to find the side of a square. The answer here will be 8√15.