Maximum current, maximum voltage
From the two statements you can see that a change in voltage caused a current change of 1A.  This means that if we consider the Thevenin equivalent of the circuit that it has an output resistance of 2V/1A = 2 ohm.  Also we can see that if we chafe R to infinity the voltage will rise to 8V (the 6V we had in statement 2plus an additional 2V by reducing the current by 1A from 1A to 0A).  The Thevenin equivalent is therefor an 8V source with a 2Ohm series resistor.

The maximum current for question (a) will be 4A with R = 0.

The maximum voltage for question (b) will be 8V with R = infinity

The values of the 24 ohm and the 6 ohm in the source are inconsistent with the measurements - that would have a Thevenin equivalent of a 24 ohm in parallel with 6 ohm which is 24/5 = 4.8 ohm.

kevin