Start with the classical equation for deflection of a guided cantilever:
d = P.L^3 / (12.E.I)
Remember that for a guided cantilever M = P.L/2
Sub into the above the engineer's bending theorem M/I = sig /y and y = Do/2
Express in terms of moment, force or stress.
eg, stress sig = (3.Do.E.d) / L^2
Job done.