The formula s= √SA/6 gives the length of the side, s, of a cube with a surface area, SA. How much longer is the side of a cube with a surface area of 1,200 square inches than a cube with the surface area of 768 square inches?

Surface area of cube = 6 * (side)^2
side^2 = surface area / 6
side = sqrt (surface area / 6)

For the cube with area 1200 square inches:
side = sqrt(surface area / 6)
side = sqrt(1200 / 6) 
side = 14.1421 inches

For the cube with area 768 square inches:
side = sqrt(surface area / 6) 
side = sqrt(768 / 6) 
side = 11.3137 inches

difference between the two lengths = 14.1421 - 11.3137 = 2.8284 inches

Based on the above calculations, the side of the cube with surface area 1200 square inches is  2.8284  inches longer than the side of the cube with surface area 768 square inches