step 1
Find the slope
Let
x -----> number of bottles
y -----> fluid ounces per bottle
so
we need two points
we take the points
(3,24) and (6,12)
the slope is equal to
m=(12-24)/(6-3)
m=-12/3
m=-4
step 2
Find the equation of the line in slope intercept form
y=mx+b
we have
m=-4
point (3,24)
substitute
24=-4*3+b
24=-12+b
b=24-12
b=12
therefore
the linear equation is
y=-4x+12
therefore
y=-4x+12
where x is the number of bottles
For x=b
substitute
y=4b+12
therefore
the fluid ounces are (4b+12)