You have $12, and need at least x more dollars to spend $30 total. So the inequality is 12 + x ≥ 30, where x is the number of dollars needed to get free delivery. To solve for x, subtract 12 from each side of the sign. x ≥ 30 - 12, x ≥ 18. You must spend at least 18 more dollars to qualify for free delivery.
