Three charges, -25 micro coulomb, +50 micro coulomb and -40 micro coulomb are placed at three coners A, B and C of a square of side 10 cm. Calculate the potential at the corner D.

Hello!
The electric potential at the distance R from a point charge Q is equal to 1/(4 pi epsi_0) Q/R, where epsi_0 is the electric constant, its value is about 9*10^(-12) F/m.  Also it is known that the potential of a system of point charges is additive (the total potential is the sum of the point potentials).
Note that the charge must be taken with the sign and in coulombs, not micro coulombs. The distance must be taken in meters. The unit for electric potential is Volt.
In this problem the three charges are given and the distances are easy to determine, 0.1 m, 0.1 sqrt(2) m and 0.1 m respectively.
So the total potential is
1/(4 pi epsi_0)*10^(-6)*(-25/0.1 + 50/(0.1 sqrt(2)) - 40/0.1) approx -1/(4 pi epsi_0)*10^(-6)*30 approx
approx -2.8*10^5 (Vol ts). This is the numerical answer.
 
 
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elepe.html