Stop being a help vampire. You either learn to do your own research or you're in the wrong class. Writing code requires you to do a LOT of research if you're going to survive in this business.
Now, again, that is a COLON, not a semicolon.
Next, if you would have Googled for "C++ for const auto", you would have found out that you're looking at a range-based for loop.
Reading the documentation on a
range-based for loop[
^], you would have seen that the expression after the colon is expected to be a "range expression", or something that returns a range of objects. This is also known as a "list", which you are expected to pass in as described in your makeSum function header.