Answer:
3.5
Step-by-step explanation:
If you multiply 3.5 for example by 2 (which is the number of gallons) you get 7 which is the cost (also a proportional relationship).
General vertex form:

Given :

Extract "spread factor" m

Complete the square


Write as a squared binomial and simplify the constant

Re-write to match signs of standard general form:

A rectangle’s area is width x height. So plug in what you know and solve. 212.5 = w x 17. To solve 212.5/17= 12.5. So the width must be 12.5.