The big "U' stands for "union".
In set notation, the union of two sets means that we combine the values into one big set. Think of a marriage (which is a union of two people). If two people marry, then they combine their stuff into one big collection (the set of all their stuff). Any duplicate items are removed.
So we have
A = {4, 6, 8}
B = {3, 4, 7, 10}
which combine to this bigger set {4, 6, 8, 3, 4, 7, 10}
now toss out the duplicate item "4" to be left with {4, 6, 8, 3, 7, 10}
If you want, you can sort the values from smallest to largest: {3, 4, 6, 7, 8, 10}
So the answer is {3, 4, 6, 7, 8, 10}
Note: the order of the set doesn't matter though it's handy to sort