$(10\%)$ A small solid gold ball $B$ is located at the center of two concentric conducting spherical shells $S_a$ and $S_b.$ The ball and the two spheres are of radius $r_0,a,$ and $b$ respectively, with $B,S_a{S}_b+Q,+Q-$QErE$-rV-r{U}_E{S}_a{S}_b{r}_0.$ Suppose the electric charges $onB,S_a,$ and $S_b$ are $+Q,+Q,$ and $-Q$ respectively. $(a)$ Use Gauss' Law $to$ find the electric field $E$ everywhere, $as$ a function $ofr,$ and draw the $E-r$ graph and the $V-r$ graph for the system.

$(b)$ What $is$ the total electric$-$field energy $U_E$ stored $in$ between the two spheres $S_a$ and $S_b?(10\%)$