Answer
175 N/C.
Work Step by Step
The intensity of the wave obeys an inverse-square distance law.
A point that is 0.200m from the source is 50 times closer to the source compared to a point that is 10.0 m away from the source. The intensity is $50^2$ times stronger at the closer point.
Recall that the intensity of any wave is proportional to the square of its amplitude. Therefore, electric field amplitude is proportional to the square root of intensity.
The amplitude is 50 times stronger at the closer point.
$$50(3.50 N/C)=175N/C$$
