The trick with this problem is that there is no trick - there's no math involved at all, just wordplay. The key is in one-time deposit; what you're looking for isn't a recurring fee, but rather a constant. Now, an equation is made up of three things:
- a variable
- a relational statement in the form of =
- a constant, even if it isn't really there, it's zero
In this case, what you're looking for is the constant in the equation; a value that doesn't change when any variable changes.
The only number in your question that fits the bill is 1200$, since it's a one-time, unchanging value. y is the total amount paid and x represents the months, which are both variables; 400 is tied to x, so it also changes based on months.