We have:

$\frac{d^{2}u}{d{y}^2}=\frac{dp}{dx}=$const

Integrating with respect to $y$ :

$\frac{\text{d}\text{e}\text{l}\text{u}}{\text{d}\text{e}\text{l}\text{y}}=\frac{1}{}\frac{dp}{dx}y+C_1$