Complex strains and strain gauges

A thin-walled cylinder is subjected to an internal pressure as shown in figure below. At a point on the outside surface of the generator, the two strain gauges recorded are, respectively, εa = 254 × 10−6 and εb = 68 × 10−6. Determine the principal stresses at the surface point. Young’s modulus and Poisson’s ratio of the cylinder are, respectively, E = 210 GPa and v = 0.28.