The change is given by the difference between how much you paid and how much you actually had to pay. For example, if you pay with a $50 bill and you had to pay $13, your change will be [tex]50-13 = 37[/tex] dollars.
In this case, we have
[tex]30 - (2.5x+2y-2) = 30-2.5x-2y+2 = 32-2.5x-2y[/tex]
So, Gabriel's mistake was to leave the negative sign to the "-2" at the end of the expression, instead of flipping it to positive "+2" when subtracting it.
