The set of solutions of $(E): y' + a(x)y = 0$ ($a\,:\mathbb{R}\,\rightarrow\,\mathbb{R}$ continuous function) is a one-dimensional vector space.
If $f(x) = e^{-\int_0^x a(t)\,\mathrm{d}t}$ is solution of $(E)$, why the general solution of $(E)$ is of the form $Cf(x)$ ($C\,\in\,\mathbb{R}$)?