Hello, I would like to know how do we get from (2) to (3):

$\displaystyle

(1)\ \ \nabla \cdot B = 0

$

$\displaystyle

(2)\ \ \frac{1}{r}\frac{\partial r B_r}{\partial r}\frac{\partial B_z}{\partial z} = 0

$

$\displaystyle

(3)\ \ rB_r = -\int_{0}^{r} r \frac{\partial B_r}{\partial z} dr

$

After (3) I know how to integrate, I just want to know how do we get from (2) to (3), if someone can do it slow with comments and as much steps.

Thanks.