\(Stress = \frac{{Force/Tension}}{{Area}} = \frac{{2mM}}{{A\left( {m + M} \right)}}g\)
\( = \frac{{2\left( {m \times 2m} \right)g}}{{A\left( {m + 2m} \right)}}\left( {M = 2m\,given} \right)\)
\( = \frac{{4{m^2}}}{{3mA}}g = \frac{{4mg}}{{3A}}\)
($\left.g=10\, {ms}^{-2}\right)$