Solution
= Solution
The damped <angular frequency> is $\Omega=\sqrt{k-\lambda^2/4}$. The oscillation envelope is proportional to $e^{-\lambda t/2}$. A decrease by a quarter leaves three quarters of the original amplitude, so $e^{-\lambda t/2}=3/4$. Therefore
$$
\boxed{\Omega=\sqrt{k-\lambda^2/4},\qquad t=\frac2\lambda\log\frac43.}
$$
This concerns the envelope; it does not ask when the instantaneous displacement first equals three quarters of a chosen value.