Question

how many coulombs must be on one object for it to be repelled with a force of 6.0 N by a second charge of 2.0uC at a distance of 1.0 m?

Answer 1

Just solve for q2 in the formula

F = (k q1 q2)/d^2

There is an interactive calculator at

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