Suppose you walk toward the rear of a moving train describe your motion as seen from a reference point on the train then describe from a reference point on the ground

Awhen a point charge of +q is placed on one corner of a square, an electric field strength of 2 n/c is observed at the center of the square. suppose three identical charges of +q are placed on the remaining three corners of the square. what is the magnitude of the net electric field at the center of the square?