After a year it's only worth (100%-15%)X = 85% of X. Let's say 0.85 X.
So January 1st 2001 it's worth 0.85 X.
At the end of that year, it's decreased by 12% again, so is worth only (100% - 12%) = 88% of what it was at the beginning of that year, which was 0.85 X.
So now it's worth (0.88)(0.85)X. (On January 1st 2002).
Then at the end of that year (which is 3 years after you bought it), it is worth 0.88 % of what it was at the start, ie. on January 1st 2002, which is
= 0.65824 X,
or 65.824 % of X, X being what you paid in the beginning.
Now that is what it is WORTH, so you're not quite done - you're asked for the percentage decrease, which is of course 100% - 65.824% = 34.176 %