$(4)$ Consider a loaded statically indeterminate beam, as shown in Figure $4.$ The symbol $k_s$ denotes the spring constant, and $k_s=E\frac{\text{I}}{L^3}.$

$($a$)$ Determine the influence line function of the spring force at point $B.$

$(15\%)$

$($b$)$ Draw the influence line and indicate the function values at the positions $\frac{x}{L}=0,\frac{1}{3},\frac{2}{3},1.(10\%)$