The formula for principal stresses is:
$$\sigma_{1,2} = \frac{\sigma_x + \sigma_y}{2} \pm \sqrt{(\frac{\sigma_x - \sigma_y}{2})^2 + \tau_{xy}^2}$$
In this case, $\sigma_x = 18$ kpsi, $\sigma_y = 0$ kpsi (since it's not given, we assume plane stress condition), and
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