2.2.5.5 ✓ Mathematica. Time used: 0.017 (sec). Leaf size: 18

ode=D[x[t],{t,2}]-4*D[x[t],t]+4*x[t]==0; 
ic={}; 
DSolve[{ode,ic},x[t],t,IncludeSingularSolutions->True]
 

\begin{align*} x(t)&\to e^{2 t} (c_2 t+c_1) \end{align*}