Open question: for balance letter of the department of the taxation asking, how much I sold my car?

I of the department of the taxation straight kept me asking to a letter, as much I my car for sold. before as I sold, my car to 6 months locked I the title notarially and gave vacant him to the customer. me asking now the department of the taxation, as much I sold it for and a bundle of other questions. I sold the car for 6000, which the chap apparent entered to 1000 into the title, therefore pay it not tax on it. I have letters, which says that the autonecessities a transmission and a machine work, which approximately 4000. in such a way cost, if I explain 5000 to them, he in the trouble received and if I 1000 as if says, are I not with them honestly. what should I do??? I am I in the trouble?