$$ \begin{equation}\label{hr1} \begin{array}[b]{rlll} y'(x^2+1)-2xy &= 0 & \vert +2xy \\ y'(x^2+1) &= 2xy & \vert \div (x^2+1) \\ y' &= 2xy \cdot \frac{1}{(x^2+1)} & \vert \div y \\ \frac{1}{y} \cdot y' &= 2x \cdot \frac{1}{(x^2+1)} & \vert \int \\ \int \frac{1}{y}~dy &= \int 2x \cdot \frac{1}{(x^2+1)}~dx & \\ \end{array} \end{equation} $$
Substitution
$$ \begin{equation}\label{nr1} \begin{array}[b]{rlll} u &= x^2+1 & \\ \frac{du}{dx} &= 2x & \vert \cdot dx \\ du &= 2x~dx & \vert \div 2x \\ \frac{1}{2x}~du &= dx & \\ \end{array} \end{equation} $$