Answer
(a) $E_x = 0$
(b) $E_x = -200~V/m$
Work Step by Step
(a) We can find $E_x$:
$E_x = -\frac{dV}{dx}$
$E_x = -(200x)~V/m$
$E_x = -(200)(0)~V/m$
$E_x = 0$
(b) We can find $E_x$:
$E_x = -\frac{dV}{dx}$
$E_x = -(200x)~V/m$
$E_x = -(200)(1)~V/m$
$E_x = -200~V/m$