Bending moment of simply supported beam subject to two equal loads at equal distance from supports.
Bending moment
$$ M_{B_2p} = {P \cdot a} $$
Calculate normal stress in simply supported beam, with known section modulus, due to bending caused by 2 symmetrical point loads.
Bending stress
$$ \sigma_{bend_{2p}} = {10 ^ 3 \cdot {M_{B_2p} \over W_{y_{section}}}} $$