Relevant data extracted from Solution 918

$\text{Area} = 60 ~ \text{in.}^2$

The largest tensile stress is at corner *B*

$\sigma_B = 320 ~ \text{psi}$

Additional compression force *P*_{2} for zero stress at corner *B*

$\sigma_B = 320 - \dfrac{P_2}{60} = 0$
$P_2 = 19,200 ~ \text{lbs}$ ← *answer*

Stress at each corner from Solution 918

$\sigma_A = -480 ~ \text{psi}$
$\sigma_B = 320 ~ \text{psi}$

$\sigma_C = 80 ~ \text{psi}$

$\sigma_D = -720 ~ \text{psi}$

Stress at each corner due to additional stress σ_{2} = -320 psi.

$\sigma_A = -480 - 320 = -800 ~ \text{psi}$
$\sigma_B = 320 - 320 = 0$

$\sigma_C = 80 - 320 = -240 ~ \text{psi}$

$\sigma_D = -720 - 320 = -1040 ~ \text{psi}$

See figure above.