Option B is the correct answer because you start with 15 gallons (y-intercept), and the car consumes or decreases at 0.03 gal/per mile (your slope or m).
3 | x+12 |=6 Start by distributing 3 to both x and 12 You get 3x+36=6 Subtract 36 from both sides You then get 3x=-30 Divide 3 from both sides getting x= -10