What two variables must remain constant when testing Charles's law?

Well, pressure has to be kept constant and so does the mass of the gas with Charles's Law. Charles's Law--V1/T1=V2/T2--can be derived from the Combined Gas Law--V1xP1/T1=V2xP2/T2--by keeping the pressure constant which in turn cancels out the pressure in the Combined Gas Law leaving you with Charles's Law. Hope that helps you!