The given expression is simplified by combining like terms. By combining the terms -20x and -7x, we get -27x. Similarly, combining x and 3y, we get 4y. Combining -12, -6, and 15, we get -3. Therefore, the simplified expression is -27x + 4y - 3. However, this does not match any of the given answer choices. Therefore, the correct answer is not provided.