The potential difference applied across a given resistor is altered so that the heat produced per second increases by a factor of 9. By what factor does the applied potential difference change?

a) Heat generated H=i²Rt=V²t/R
H'=9H = V'²t/R
9H/H = V'²/V²
9 = V'²/V²
V' = 3V
So potential difference also increases by 3 times.
b) The ammeter is connected in series and has negligible resistance.
Total resistance = 4+2 = 6Ω
Ammeter reading = 12/6 = 2A