Which expression demonstrates the use of the commutative property of addition in the first step of simplifying the expression (–3 + i) + (18 + 5i)?

The first step of simplifying the expression would be summing up the "i"s. They are separate ("+i" and "+5i") but you can still sum them up though they are apart. This is thanks to the commutative property of addition which allows summing the terms of an expression with several additions in whatever order it is prefered. Commutative means that the terms can be interchangeable.