Similarities and differences between series and parallel circuit

A Series Circuit can be described as:

A circuit in which:

AND

A Parallel Circuit can be described as:

A circuit in which:

a charge 3q is at the origin, and a charge −2q is on the positive x axis at x=a. part a where on the x-axis would you place a third charge so it would experience no net electric force? enter an expression for the exact answer, which will involve a square-root sign. (do not enter an approximate decimal answer.)

the independent variable, or the one that is intentionally manipulated is the speed.