Since the charge experiences a repulsive force, the sign of the other charge is positive.
<br>(same charges repells)
<br>we have, F = AQ1q2/R^2 .....(1)
<br>where A = 9x10^9
<br>F = 9x10-9 N
<br>Q1= 2x10-12 C
<br>R = 10 cm = 10x10^-2m = ...
Very detail answer with easy to understand steps. It will enable you to do similar problems yourself.