To follow up Richard's comments a bit...
Your interface is confusing. Yep, you've managed to give the class two ways of doing the same stuff - i.e. you can write something like:
Calculator c( 27, 84 );
std::cout << c.average() << std::endl;
or:
Calculator c;
c.setFirst( 27 );
c.setSecond( 84 );
std::cout << c.average() << std::endl;
Pick one and stick with it, your code will be cleaner and easier to read that way. Given my highly rational hatred of getters and setters stick with constructing an object and then using it, don't build it then twiddle it into shape before you can use it.