Let the Addison's candy count be a and Ronny's be r.
Then a = r - 15. This is an equation.
If you had a second equation, you could replace a with the expression (r - 15), which in itself is not an equation.
The formula of the line is
y=mx + b
The likes goes through the point (0,0)
that means that y-int = 0, b=0
y=mx
So we need to find only slope.

48/14 so you just simplify that for gallons/minutes. something that goes into both of those numbers is 2 so you would have 24/7 when you divide by 2. that’s your answer.
24/7
You would need more details like the y values or something. Rate of change is slope though if that helps you enough to figure it out