$1142 – 921 = 821 $
For solving it fast logically, first find the preconditions. We can see that one number is 921, means the base must be greater than 9. As the base is greater than 9, subtracting last two digits 4221 gives us 21 which is obvious. when we see 18, we take "borrow" from left hand side. And hence 119 is performed, but the result is 8 ! In decimal 179 gives us 8. Hence $11_x$ must be $17_{10}$
And we know that 11 in Hexadecimal gives 17 in decimal. Hence we conclude that base is 16.
Now perform quick subtraction :

1 
6 
7 
4 
 

7 
8 
8 

0 
Take Borrow (5+16)7=14=E 
Take borrow
(6+16)8=14=E

Take borrow (4+16)8=12=C

Hence the ans is $EEC$