two point charges $(+10$nC and $-10$ nC$)$ are located $8\mathrm{.}00$ cm apart. let u $=0$ when all the charges are seperated by infinite distances. a$)$ what is the potential energy for these two charges? b$)$ what is the potential energy if the third point charge is q $=-4\mathrm{.}2$nC is placed at point b$?$