Juan and Maria had been engaged for 5 years and since they were very happy together, Juan asked him: Maria, will you marry me?
To which Maria replied: "I would not be lying if I told you that I cannot not tell you that it is impossible to deny that if I believe it is true that it is still false that we will not get married."
What have you answered?
The answer is that they will get married. The way to see it is to simplify the phrase little by little to eliminate the double denials that become affirmations.