I am struggling to answer this question:

A timber beam having a rectangular cross section 240mm x 80mm is simply supported over a span of 4m. If the permissible bending stress is 5N/mm^2, determine the maximum allowable uniformly distributed load that the beam may carry, applied perpendicular to the x-x axis?

What I have done is:

Ixx = bd^3/12 = 80 x 240^3/12 = 92160000 mm^4 or 9216.0.0 x 10^3 mm^4

Then, using section modulus I done:

z = I/Y = 92160000/120 = 768000 mm^3

Now do not know what to do