for this you simply plug it in.
a=6 b=4 c=8
so, 2(a) would be 2(6) because a=6
2 times 6 is 12
in the case of 3(b)-c you also plug it in
3(4)-8
12-8
4
so i thought that the 2a and the 3b-c were one expression becuase your question was not plurl but you do not have a relationship between them, if it is one expression you can comment on my answer and i can resolve this for you.
