HEy, What's up?
the fastest way for this is:
change 888 base 9 +555 base 6 to base 10
then change it to base 11
and the trick is if aaa base ( a + 1) = ((a+1)to the power of 3 minus 1) base 10
so 888 base 9 = 9 to the power of 3 is 729; minus 1 is 728 base 10
555 base 6= 6 cube is 216 minus 1 is 215 base 10
728+215=943base 10
change 943 base 10 to base 11
get the answer!