Given,
The supply voltage, V=121.7 V
The current drawn by the oven, I=5.7 A
From Ohm's law, the voltage across a load is directly proportional to the current through the load.
Thus the voltage across the oven is given by,

Where R is the resistance of the oven when it is heated to the maximum temperature.
On rearranging the above equation,

On substituting the known values,

Thus the resistance of the oven when it's fully heated is 21.35 Ω