To simplify the expression, we substitute X with 202:

15(202) - 12(202) + 398 - 202
= 3030 - 2424 + 398 - 202
= 606 + 398 - 202
= 1002 - 202
= 800

Therefore, 15x - 12x + 398 - X when X = 202 is equal to 800.