$y\prime \prime \+4 y\prime \+3 y\=0$

$\frac{{\ⅆ}^2}{\ⅆx^2}y\left(x\right)\+4\left(\frac{\ⅆ}{\ⅆx}y\left(x\right)\right)\+3y\left(x\right)\=0$

$\mathrm{DEtools}\left[\mathrm{odeadvisor}\right]\left(\right)$

$\left[\left[\mathrm{\_2nd\_order}\,\mathrm{\_missing\_x}\right]\right]$

$$\begin{gathered} \mathrm{infolevel}\left[\mathrm{dsolve}\right]≔3\: \\ \mathrm{dsolve}\left(\right)\; \\ \mathrm{infolevel}\left[\mathrm{dsolve}\right]≔0\: \end{gathered}$$

Methods for second order ODEs:
--- Trying classification methods ---
trying a quadrature
checking if the LODE has constant coefficients
<- constant coefficients successful

$y\left(x\right)\&equals;\mathrm{\_C1}{\&ExponentialE;}^{-x}\&plus;\mathrm{\_C2}{\&ExponentialE;}^{-3x}$