6V healthy

U 1= 1.5V, because it is in series, the current is constant U2 = 6V- 1.5v = 4.5v ... Suppose I= 1.5A (it will be an integer later). According to ohm's law, when the slider is in C, R 1= 1, R2=3, R2= 1.5. And because the total u = U 1+U2, I always have R = I1R1.