Having established the state of stress at $D$, we proceed to the computation of the principal stresses. We can use either the plane stress transformation equations directly or we can use the Mohr's circle transformation.

For this example, we will use Mohr's circle. We first plot two points: the state of stress on the $x$ face and the state of stress on the $y$ face. From these two points, we construct the circle, which represents combinations of $\sigma$ and $\tau$ at all possible orientations.

For this case, the center of the circle is $C=-6.29 ksi$ and the radius is $R=6.89 ksi$.