#### Answer

$E = 1200~N/C$

#### Work Step by Step

Let $V$ be the volume of the space.
We can find the strength of the electric field $E$:
$\frac{U}{V} = \frac{1}{2}\epsilon_0~E^2$
$E^2 = \frac{2U}{V\epsilon_0}$
$E = \sqrt{\frac{2U}{V\epsilon_0}}$
$E = \sqrt{\frac{(2)(50\times 10^{-12}~J)}{(0.020~m)^3(8.854\times 10^{-12}~F/m)}}$
$E = 1200~N/C$