Answer:
122.84 J
Step-by-step explanation:
Since plate is square, area, A is given by

The distance between plates, d, is given in the question as 2mm=0.002m
Charge on plate, Q, as given in the question is 240

Assuming mica dielectric constant, k of 7
Capacitance, C is given by
C=

Stored energy, E is given by
E=

Therefore, the stored energy is 122.84 J