Euler's equation for an incompressible fluid

Question

I am trying to solve the following problem:

I rearranged the Euler equation to find grad(p) and then I did -u.grad(p) to see if i could simplify this to get the LHS

However i am very stuck on what happens when i apply -u.grad(p) i would appreciate any hints to help me

Answer 1

Hint

$\frac{D\phi}{Dt}=\frac{\partial \phi}{\partial t}+\vec{u}.\nabla \phi$

$=\vec{u}.\nabla \phi$ since $ \phi$ is

time independent.