Here, heat is given as 400 J per second.
So, P= 400 J / 1 s
Suppose the equation used is P= V2/R, replacing the values you get
400 = V2/9.
Hence, V2= 400 x 9
Therefore, V will be √(400 x 9) = 20 x 3=60V
Thus, potential difference applied across the resistance is 60V.