Question

the following point charges are placed on the x-axis: +2.0μC at x=20cm,-3.0μC at x=30cm and -4.0μC at x=40cm. What is the potential at x=0?

Answer 1

Just use the formula for each point and add 'em up. They're all in a nice straight line, so there are no vector calculations involved.

Take a peek at

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/potpoi.html