You know she bought one drink for $2.99, TWO bags of chips for $1.29 each, and some gallon quantity, (X), of gasoline at $2.79.
$2.99+(2 x $1.29) + $2.79x = y, where y is the total amount of money spent at the convenience store.
$2.99+$2.58+$2.79x = y
$5.57 + $2.79(x) = y
