如图所示，两个点电荷＋q和－3q，相距为d.试求：<br/> <img src="https://nimg.ppkao.com/2019-05/wangjing/2019050816220578760.jpg?sign=3e0c86ec429f9a13ba47114f23bb0796&t=62d5ca33"/> 在它们的连线上电场强度<img src="https://nimg.ppkao.com/2019-05/wangjing/2019050816214796065.jpg?sign=391b3ec83ecbcbf7b2272a3ea09960d7&t=62d5ca33"/>=0的点与电荷为＋q的点电荷相距多远？