The area of the blue and the white square together is Area_{big}which is the length of the base (29m) times the length of the height (16m).

The area of just the white square together is Area_{small}which is the length of the base (25m) times the length of the height (12m).

e the Taksmall area away from the big area and you are left with just the blue area

Dividing the blue area by the area of one tile gives the number of tiles needed to cover the area.