Linearized pendulum

${}^{\text{'}\text{'}}+\frac{c}{m}{}^{\text{'}}+\frac{g}{L}=\frac{1}{mL}h(t)$

Angular displacement $(t),$ mass $m,$ damping constant $c,$ length $L,$ gravitational constant $g,$ external tangential force $h(t).$

Given a linearized and undriven pendulum with a mass of $2$ kg $,$ if $c=0$ and $L=2m,$ find the angular displacement at time $t=3s.(0)=1$ and ${}^{\text{'}}(0)=1$