Two positive point charges, QA$=2\mathrm{.}75\backslash$times $10-6$C and QB$,$ whose magnitude is unknown, are situated on an x$-$axis as shown in the figure. Note that the positive direction is to the right. A small positive test charge $($black dot$)$ is placed between the two charges, so that rA$=0\mathrm{.}750$m and rB$=0\mathrm{.}250$m$.$What is charge QB so that the total electric field at the position of the positive test charge will be zero?

K + Q A r A VB * + QB +x