Be careful not to confuse the ∪ for union with the U 
for the universal set. Try to make them a little different.
It's a shame that the set theorists did that to us! Anyway:

Substitute the sets for the letters:

({20,40,60,80} ∪ {30,60,90})C

∪ stands for union, which means to combine all the elements together
into one set.  If an element is in both sets, only include it once in
the union. So we have as our next step:

({20,30,40,60,80,90})C

The C as a superscript means "complement", which tells us not to use 
any elements in the set, and instead use all the other elements of the
universal set that are not in that set.  So we end up with this set:

{10,50,70}

Edwin