2.2.7.7 ✓ Mathematica. Time used: 0.021 (sec). Leaf size: 19

ode=D[x[t],{t,2}]+3*D[x[t],t]==0; 
ic={}; 
DSolve[{ode,ic},x[t],t,IncludeSingularSolutions->True]
 

\begin{align*} x(t)&\to c_2-\frac {1}{3} c_1 e^{-3 t} \end{align*}