= Solution
For small horizontal displacement $x_i\simeq l\theta_i$, the gravitational restoring force is $-mg\theta_i\simeq-(mg/l)x_i$. A spring between adjacent bobs has extension equal to their displacement difference; <Hooke's law> pulls each bob towards its neighbour's displaced position. Thus comparison with the stated equations gives
$$
\boxed{a=\frac gl,\qquad b=k.}
$$
The small-angle approximation makes the bob motion horizontal to first order; its vertical shift changes spring geometry only at higher order. Multiplying all force equations by minus one puts the positive restoring coefficients on the displayed right-hand side.
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