What multiples to -112 and adds to 6

How do i simply this expression (quadratic formula basis) on a ti-84 or normal calculator?

Which of the following scenarios demonstrates an exponential decay

The water company charges a residential customer $41 for the first 3,000 gallons of water used and $1 for every 200 gallons used over 3000 gallons. if the water bill was $58 how many gallons of water were usedanswersa 3400 gallonsb 6000 gallonsc 6400 gallons d 7000 gallons