Explanation :
Hoop stress $6c=\frac{P\cdot d}{2t}=\frac{2\times14}{2\times0.05}=280\;MPa$
longitudal stress $6t=\frac{P\cdot d}{4t}=\frac{2\times14}{4\times0.05}=140\;MPa$
$6_3$= 0
$Max.\;shear\;stress=Max\left\{\left|\frac{6_1-6_2}2\right|\cdot\left|\frac{6_2-6_3}2\right|\cdot\left|\frac{6_3-6_1}2\right|\right\}$
=Max {70,70,140}
= 140 MPa .