In the figure four particles form a square with edge length $a=4\mathrm{.}54{10}^{-2}m.$ The charges are $q_1=q_4=2\mathrm{.}91{10}^{-15}C$ and $q_2=q_3=q.($a$)$ What is $q$ if the net electrostatic force on particle $1$ is zero? $($b$)$ Is there any value of $q$ that makes the net electrostatic force on each of the four particles zero?

a$)$ Number

Units

b$)$