$=270 - 4.5 x$
Work Step by Step
We know that the balance on the card starts at 270 dollars and decreases by 4.5 dollars per lunch. Thus, the coefficient in front of x is -4.5, making the expression: $=270 - 4.5 x$ It is important to note that this makes sense for most positive integers, given that the overall balance does not become negative. Thus, as long as x is a small enough positive integer, the expression will make sense.