Prob $7.$ The rigid bar below is supported an aluminum rod $AC(E_{\text{a}\text{l}\text{u}\text{m}\text{.}}=70$GPa$)$ with a diameter of $80$ mm and a steel rod$(E_{\text{s}\text{t}\text{e}\text{e}\text{l}}=200$GPa$)$ with a diameter of $60$ mm $.$ A load of $900$ kN is applied as shown. $($a$)$ Determine the stresses in both rods. $($b$)$ Determine the vertical displacement of the point at which the force is applied.