The earths magnetic field, like any magnetic field, stores energy. The maximum strength of the earths field

Question:

The earth’s magnetic field, like any magnetic field, stores energy. The maximum strength of the earth’s field is about 7.0 × 10-5 T. Find the maximum magnetic energy stored in the space above a city if the space occupies an area of 5.0 × 108 m2 and has a height of 1500 m.