$(1000)_{3} -(0121)_{3}$. This would be much like subtraction in decimal system , only with one small change . While doing a borrow , instead of 10 , 3 will be borrowed.
The subtraction can be seen like :-
$(0223)_{3} - (0121)_{3}$[This is after doing the borrows.Kindly do it once , if not able to get i'll show the entire steps.]
Thus the difference will be $(0102)_{3} = 1*3^{2} + 2*3^{0} = 9+2 = (11)_{10}$.
Let's confirm this by converting this to decimal system.
$(1000)_{3} = (27)_{10}$
$(0121)_{3} = (16)_{10}$
$(27)_{10} - (16)_{10} = (11)_{10}$.