$($a$)$ Find the potential at point P$.($q $=2\mathrm{.}5$ x $10-9$C$,$ a $=50$ cm$)($b$)$ How much work by an external force is required to move a charge of $-3$C to point P$.($c$)$ If the particle $($mass $=5\mathrm{.}0$g$)$ were released from rest at point P$,$ what would be the value of its initial acceleration?