In a horizontal $2-$dimensional stress field $($parallel to the Earths surface$)$ calculate the

principal stresses ${}_1$ and ${}_2$ and their bearing if:

${}_{\text{N}\text{o}\text{r}\text{t}\text{h}}=21\mathrm{.}1$MPa,${}_{\text{E}\text{a}\text{s}\text{t}}=18\mathrm{.}2MPa{T}_{\text{N}\text{o}\text{r}\text{t}\text{h}-\text{E}\text{a}\text{s}\text{t}}=1\mathrm{.}2$MPa