An 8-pound bag of apples costs $17.50. what is the unit price?

8 pound bag full of apples = $17.50 

The unit price is the amount we need to pay per bag.

Now, we need to set up a proportion. 

8/17.50 = 1/x

Cross multiply.

8*x = 8x
17.50 * 1 = 17.50

8x = 17.5

Divide

8x/8 = 17.5/8
x= 17.5/8

x= 2.18

2.18 dollars per pound. 

The vertex of the quadratic function is (1.5, -.75)