Answer
The magnitude of the electric field strength between the electrodes is $600~V/m$
Work Step by Step
We can find the electric field strength between the electrodes:
$E = -\frac{\Delta V}{\Delta x}$
$E = -\frac{(35~V-20~V)}{0.025~m}$
$E = -\frac{(15~V)}{0.025~m}$
$E = -600~V/m$
The magnitude of the electric field strength between the electrodes is $600~V/m$.