I'll do no. 2 for you, you should then be able to do the rest.
2) 321(6) + 1255(6)
Step by step:
5+1 = 6 = 0(6) with a 1 carried:
1
1255
321
----
0
1+5+2 = 8 = 2(6) with a 1 carried:
1
1255
321
----
20
1+3+2 = 6 = 0(6) with a 1 carried:
1
1255
321
----
020
1+1 = 2 = 2(6):
1255
321
----
2020(6)
Check: 321(6) = 3*6^2+2*6+1 = 121(10)
1255(6) = 1*6^3+2*6^2+5*6+5 = 323(10)
121(10)+323(10) = 444(10)
2020(6) = 2*6^3 + 2*6 = 432 + 12 = 444(10) (ok)