# Substituting principal stress angle into the plane stress transformation equations

When one substitutes the angle -23.7 into the plane stress transformation equation for SigmaX' you indeed get -46.42 MPa (as shown in the images). However, if you substitute this same angle into the plane stress transformation equation for SigmaY', you yield the other principal stress of 116.4 MPa. My question is how can you conclude the angle -23.7 corresponds to SigmaX' rather than SigmaY' ?