Assuming the same cross$-$section for all members, determine the minimum cross$-$section area if the modulus of material is $180$ GPa $,$ and the allowable vertical displacement at $D$ is $6$ inches.

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I have obtained the forces in AB as $8\mathrm{.}2$ kips, AG as $4\mathrm{.}5$ kips and FG as $-11\mathrm{.}60$ Kips. How do i do the second part?