(6,16800),(10,5600)
slope = (5600 - 16800) / (10 - 6) = -11200/4 = -2800
y = mx + b
slope(m) = -2800
use either of ur points...(10,5600)...x = 10 and y = 5600
now we sub into the formula and find b, the y int
5600 = -2800(10) + b
5600 = -28000 + b
5600 + 28,000 = b
33600 = b
so the equation is : y = -2800x + 33600...with y being the number of gallons and x being the number of weeks.....and if all of the water is used up in the dry season, then to find the number of weeks, we have to let y = 0
0 = -2800x + 33600
2800x = 33600
x = 33600 / 2800
x = 12 <== the dry season is 12 weeks
