When the current through the sliding rheostat is 2A and the power supply voltage is U= 12V, the resistance of R0 is the largest, and R0=U/(I-I sliding) =12V/(3A-2A) =12Ω.
When the power supply voltage U=6 and the sliding rheostat is the maximum, the resistance of R0 is the minimum, and the current through the sliding rheostat I-Slip = U/R-Slip = 6V/10ω = 0.6A; The current I0 through I0 = I-I slip = 3a-0.6a = 2.4a;; r0 = U/Io = 6V/2.4A = 2.5ω