Answer:
s=-2
Step-by-step explanation:
Explanation in picture
standard from Ax +By = C
y = 4x+10
subtract 4x from each side
-4x+y = 10
or 4x - y = -10
Yes, the cost is proportional to the number of water bottles sold. 6 water bottles/$4.00 = 1.5 and 15 water bottles/$10.00 = 1.5.