If the image of the function y = 21-x + 3 is shifted one unit to the left and four units to the down, then it is symmetric about the X axis, the analytic expression of the function is___ ．

If the image of the function y = 21-x + 3 is shifted one unit to the left and four units to the down, then it is symmetric about the X axis, the analytic expression of the function is___ ．

The function image is moved one unit to the left and four units to the down, that is, x + 1 is used to replace x, y + 4 is used to replace y, and the corresponding function of the new image is obtained. Therefore, the image of the function y = 21-x + 3 is moved one unit to the left and four units to the down, then x becomes x + 1, y → y + 4, and the image of Y + 4 = 21 - (x + 1) + 3 is obtained, that is, y = (1