So, let's say the soft drink equals y and the sandwich equals x.
x + y = 9.9
The sandwich costs 3.5 times as much as the soft drink
x = 3.5y
Therefore, we can replace x with 3.5y since they're the same thing
3.5y + y = 9.9 <-- That's the equation :)
I would say you cannot use a rectangle. Or a circle. One of those.
-.38, -0.08 , -.6 or -3/5 ,0.04