Section Modulus calculation and optimization

Section modulus is the resistance against the force on a member which is defined by the formula S = I/y where I is the area moment of inertia and y is the distance between the central axis and extreme end. 

here I = 2.8x10^5 mm^4 and y = 438.4 mm

Now, S= I/y

          S = 2.8x10^5/438.4

          S = 638.686 mm^3