Write a linear function that relates cost y to ounces x.

Note that y appears to change by 1 when x changes by 4.......so the slope of our function is 1/4

And using one of the points (12, 2.5)   we have  ...in point-slope form ....

y - 2.5 = (1/4)(x - 12)      add 2.5 to both sides

y = (1/4)(x - 12) + 2.5     distribute the 1/4 across the terms in the parentheses

y = (1/4)x - 3 + 2.5         simplify

y = (1/4)x - 1/2             so when x = 8, y should = 1.5...let's see if that's true

y = (1/4)(8) - 1/2

y = 2 - 1/2 

y = 3/2 = 1.5

So....our equation is correct