Suppose,the #4# like charges are present at #A,B,C,D# and #AB=BC=CD=DA=0.2m#

We are considering forces on #B#;so due to #A# and #C# force(#F#) will be repulsive in nature along #AB# and #CB# respectively.

due to #D# force (#F'#) will also be repulsive in nature acting along diagonal #DB#

#DB=0.2sqrt(2)m#

So,#F=(9*10^9*(16*10^-6)^2)/(0.2)^2 =57.6N#

and #F'=(9*10^9*(16*10^-6)^2)/(0.2sqrt(2))^2=28.8N#

now,#F'# makes an angle of #45^@# with both #AB# and #CB#.

so,component of #F'# along two perpendicular direction i.e #AB# and #CB# will be #28.8 cos 45#

So,we have two forces of #(57.6+28.8 cos 45)=77.95N# acting perpendicular to each other on the charge at #B#

So,net force on the charge at #B# is #sqrt(77.95^2 +77.95^2)=77.95 sqrt(2)=110.24N# making an angle of #45^@# w.r.t #AB# and #CB#