Find the slope of the line that passes through the two
points.
(12, 15 ) and (5,7)

i assume that by 5 you mean 5 gallons.

(20)(0.25) = 15

removing 15 gallons reduces the tank from 20 to 5.

you should talk someone who owns the website about it. you did not deserve to have all that hard work and time taken away. id watch videos of little puppies to cool you down, and maybe some nacho cheese doritos.