Solution
= Solution
With $Z'(0)=k$, the linearized kinematic condition gives
$$
\cos\frac{\pi x}{L}\cos\frac{2\pi y}{L}H'(t)
=k\cos\frac{\pi x}{L}\cos\frac{2\pi y}{L}F(t).
$$
Hence
$$
\boxed{\eta(x,y,t)=\cos\frac{\pi x}{L}
\cos\frac{2\pi y}{L}H(t)},
\qquad
\boxed{H'(t)=kF(t)}.
$$