variation of parameters

$$\begin{array}{c}\begin{array}{rl}{y}^{\prime }+p(t)y& =0\\ y^{\prime }\frac{1}{y}& =-p(t)\\ (\ln |y|{)}^{\prime }& =-p(t)\\ y& =C{e}^{-\int p(t)dt}\end{array}\end{array}$$