U = {1,2,3,4,5,6,7,8}, A={2,4,5}, B = {1,4,7,8}, C = {3,6,8}. (Use the roster method to write the set. enter EMPTY for the empty set)
A U (B U C)
step 1
Find out B U C
we have
B = {1,4,7,8}, C = {3,6,8}
so
B U C={1,4,7,8} U {3,6,8}={1,3,4,6,7,8}
step 2
Find out A U (B U C)
A U (B U C)={2,4,5} U {1,3,4,6,7,8}={1,2,3,4,5,6,7,8}=set U
therefore
the answer is
{1,2,3,4,5,6,7,8}