Answer
$y \cdot e^{\int P(x)dx} = \int Q(x) \cdot e^{\int P(x)dx}\,dx + C$, where $\text{IF} = e^{\int P(x)dx}$ is the integrating factor.
Explanation
Multiplying by the integrating factor transforms the left side into the exact derivative $\frac{d}{dx}(y \cdot \text{IF})$.