$1.$ For the three$-$story shear building shown in figure below. a$)$ Determine the natural vibration frequencies and modes; express the frequencies in terms of $\backslash ($ m $\backslash ),\backslash ($ E I $\backslash ),$ and $\backslash ($ h $\backslash ).$ b$)$ Determine the free vibration responses of the structure shown above if it is displaced as shown in the figures below $($case $\backslash ($ a$,$ b $\backslash ),$ and c$)$ and then released. $($a$)($b$)($c$)$