Answer
The magnitude of the electric field strength is $20,000~V/m$ and it points straight down.
Work Step by Step
The electric field points from positions of higher potential to positions of lower potential. Therefore, the electric field points straight down.
We can find the electric field strength:
$E = -\frac{\Delta V}{\Delta x}$
$E = -\frac{400~V}{0.02~m}$
$E = -20,000~V/m$
The magnitude of the electric field strength is $20,000~V/m$ and it points straight down.
