Three point charges

q$1,$

q$2,$

and

q$3$

are situated at three corners of a rectangle as shown in the diagram below. Here

q$1=+2\mathrm{.}00$C$,$

q$2=2\mathrm{.}00$C$,$

q$3=+3\mathrm{.}00$C$.$What is the electric potential at the free corner where there is no charge?

$?$What charge should be placed at the free corner for the electric potential at the center of the rectangle to be zero? Include both magnitude and sign if applicable