Ok for the new dimensions I made an error my bending moment is 8.4 not 8.33 for a start. For the new dimensions 100mmx240mm I got the I value of 1.152x10^-4. This was in turn plugged into the equation 8.4x100x10^-3/1.152x10^-4 the result was 7291.666667 which I think is 7.29x10^8
The other question I am trying to solve is this At the point of maximum stress sketch a graph of the stress
distribution through the thickness of the beam, indicating which are
tensile and compressive stresses.
It was using your equation that gave me the 6.29 results. really hoping we can solve these last two today...........