Problem $2$: Textbook Problem $5\mathrm{.}7$; Note: Torsional bars $K_2$ and $K_3$ are in series. Use same formula as used for linear springs in series.

$5\mathrm{.}7.$ Find the value of $K_{eq}$ in Figure P$5\mathrm{.}7($b$)$ such that the relationship between ${}_a(t)$ and $$

is the same as for Figure P$5\mathrm{.}7($a$).$

Figure P$5\mathrm{.}7$