Cost = ##$3x + $4.50##

We do not know the cost of each book of stamps, but whatever it was, Spencer bought 3 of them.

Let’s call the value we don’t know, ##$x##.

The money spent on the books of stamps is: ##3 xx $x = $3x##

He also spent money mailing the package, but that amount we do know.

His total cost is therefore the money spent on the books as well on the mailing. Cost = ##$3x + $4.50##

This is an expression which we cannot simplify.